The disks appear to be the same size in both sites, and all disks are being replicated. The vmfs version and block size are different, but that shouldn't matter. Interestingly, I tried to replicate two more servers and get the same error. Here is a snip from the log at the target VRA side. I'm not saying this is responsible for the error in the GUI, but it is the only 'mismatch' term I can find:
2013-07-08T00:39:52.686+01:00 [7F5ED952B700 verbose 'Db'] SQL code: SELECT * FROM ReplicationGroup WHERE (groupID == 'GID-558eb100-f9b1-4d2d-89ac-ae017aceef67');
2013-07-08T00:39:52.686+01:00 [7F5ED952B700 verbose 'Db'] SQL code: SELECT * FROM ReplicatedDisk WHERE (groupID == 'GID-558eb100-f9b1-4d2d-89ac-ae017aceef67') AND (isUnconfigured == 0);
2013-07-08T00:39:52.686+01:00 [7F5ED952B700 info 'Delta'] Configured disks for group GID-558eb100-f9b1-4d2d-89ac-ae017aceef67:
2013-07-08T00:39:52.686+01:00 [7F5ED952B700 info 'Delta'] RDID-2772fbab-d0d7-4649-aeec-ac8f2b7fd98a
2013-07-08T00:39:52.686+01:00 [7F5ED952B700 info 'Delta'] RDID-744069f7-5491-4d2f-822c-358588a69466
2013-07-08T00:39:52.686+01:00 [7F5ED952B700 info 'Delta'] RDID-e85ec2f7-98f2-4d1b-a654-e841026d0d0e
2013-07-08T00:39:52.686+01:00 [7F5ED952B700 verbose 'Db'] SQL code: SELECT * FROM ReplicatedDisk WHERE (diskID == 'RDID-2772fbab-d0d7-4649-aeec-ac8f2b7fd98a');
2013-07-08T00:39:52.687+01:00 [7F5ED952B700 verbose 'Db'] SQL code: SELECT * FROM DiskInstance WHERE (diskInstanceKey == 209);
2013-07-08T00:39:52.687+01:00 [7F5ED952B700 verbose 'Db'] SQL code: SELECT * FROM DiskInstance WHERE (diskInstanceKey == 209);
2013-07-08T00:39:52.687+01:00 [7F5ED952B700 info 'Setup'] Created ActiveDisk for DiskID: RDID-2772fbab-d0d7-4649-aeec-ac8f2b7fd98a Base path: /vmfs/volumes/51c47e89-58253ce9-1a7b-00221968514a/SVR-SQL2/SVR-SQL2.vmdk curPath: /vmfs/volumes/51c47e89-58253ce9-1a7b-00221968514a/SVR-SQL2/SVR-SQL2.vmdk diskType: Basic
2013-07-08T00:39:52.687+01:00 [7F5ED952B700 verbose 'HostPicker'] AffinityHostPicker choosing host host-44203 for context '[] /vmfs/volumes/51c47e89-58253ce9-1a7b-00221968514a/SVR-SQL2'
2013-07-08T00:39:52.690+01:00 [7F5ED952B700 info 'Host'] NFC creating session to target: 1.2.3.4 port: 902
2013-07-08T00:39:52.998+01:00 [7F5ED9D8C700 verbose 'SessionManager'] vmodl.query.PropertyCollector.waitForUpdatesEx: authorized
2013-07-08T00:39:53.061+01:00 [7F5ED952B700 info 'RemoteDisk'] RemoteDisk: Opened path (/vmfs/volumes/51c47e89-58253ce9-1a7b-00221968514a/SVR-SQL2/SVR-SQL2.vmdk) (longContentId='4c86b41cb81195e4387bb23bfffffffe')
2013-07-08T00:39:53.061+01:00 [7F5ED952B700 warning 'RemoteDisk'] longContentId mismatch: Expected '' Actual '4c86b41cb81195e4387bb23bfffffffe' (path=/vmfs/volumes/51c47e89-58253ce9-1a7b-00221968514a/SVR-SQL2/SVR-SQL2.vmdk) (disk=RDID-2772fbab-d0d7-4649-aeec-ac8f2b7fd98a)
2013-07-08T00:39:53.062+01:00 [7F5ED952B700 verbose 'Db'] SQL code: SELECT * FROM ReplicatedDisk WHERE (diskID == 'RDID-744069f7-5491-4d2f-822c-358588a69466');
2013-07-08T00:39:53.062+01:00 [7F5ED952B700 verbose 'Db'] SQL code: SELECT * FROM DiskInstance WHERE (diskInstanceKey == 210);
2013-07-08T00:39:53.062+01:00 [7F5ED952B700 verbose 'Db'] SQL code: SELECT * FROM DiskInstance WHERE (diskInstanceKey == 210);
2013-07-08T00:39:53.062+01:00 [7F5ED952B700 info 'Setup'] Created ActiveDisk for DiskID: RDID-744069f7-5491-4d2f-822c-358588a69466 Base path: /vmfs/volumes/51c47e89-58253ce9-1a7b-00221968514a/SVR-SQL2/SVR-SQL2_1.vmdk curPath: /vmfs/volumes/51c47e89-58253ce9-1a7b-00221968514a/SVR-SQL2/SVR-SQL2_1.vmdk diskType: Basic
2013-07-08T00:39:53.062+01:00 [7F5ED952B700 verbose 'HostPicker'] AffinityHostPicker choosing host host-44203 for context '[] /vmfs/volumes/51c47e89-58253ce9-1a7b-00221968514a/SVR-SQL2'
2013-07-08T00:39:53.065+01:00 [7F5ED952B700 info 'Host'] NFC creating session to target: 1.2.3.4 port: 902
2013-07-08T00:39:53.196+01:00 [7F5ED952B700 info 'RemoteDisk'] RemoteDisk: Opened path (/vmfs/volumes/51c47e89-58253ce9-1a7b-00221968514a/SVR-SQL2/SVR-SQL2_1.vmdk) (longContentId='a5e67f9090bde17decfa6082fffffffe')
2013-07-08T00:39:53.196+01:00 [7F5ED952B700 warning 'RemoteDisk'] longContentId mismatch: Expected '' Actual 'a5e67f9090bde17decfa6082fffffffe' (path=/vmfs/volumes/51c47e89-58253ce9-1a7b-00221968514a/SVR-SQL2/SVR-SQL2_1.vmdk) (disk=RDID-744069f7-5491-4d2f-822c-358588a69466)
2013-07-08T00:39:53.196+01:00 [7F5ED952B700 verbose 'Db'] SQL code: SELECT * FROM ReplicatedDisk WHERE (diskID == 'RDID-e85ec2f7-98f2-4d1b-a654-e841026d0d0e');
2013-07-08T00:39:53.196+01:00 [7F5ED952B700 verbose 'Db'] SQL code: SELECT * FROM DiskInstance WHERE (diskInstanceKey == 211);
2013-07-08T00:39:53.197+01:00 [7F5ED952B700 verbose 'Db'] SQL code: SELECT * FROM DiskInstance WHERE (diskInstanceKey == 211);
2013-07-08T00:39:53.197+01:00 [7F5ED952B700 info 'Setup'] Created ActiveDisk for DiskID: RDID-e85ec2f7-98f2-4d1b-a654-e841026d0d0e Base path: /vmfs/volumes/51c47e89-58253ce9-1a7b-00221968514a/SVR-SQL2/SVR-SQL2_2.vmdk curPath: /vmfs/volumes/51c47e89-58253ce9-1a7b-00221968514a/SVR-SQL2/SVR-SQL2_2.vmdk diskType: Basic
2013-07-08T00:39:53.197+01:00 [7F5ED952B700 verbose 'HostPicker'] AffinityHostPicker choosing host host-44203 for context '[] /vmfs/volumes/51c47e89-58253ce9-1a7b-00221968514a/SVR-SQL2'
2013-07-08T00:39:53.200+01:00 [7F5ED952B700 info 'Host'] NFC creating session to target: 1.2.3.4 port: 902
2013-07-08T00:39:53.338+01:00 [7F5ED952B700 info 'RemoteDisk'] RemoteDisk: Opened path (/vmfs/volumes/51c47e89-58253ce9-1a7b-00221968514a/SVR-SQL2/SVR-SQL2_2.vmdk) (longContentId='55fe6edc3dabc41e7034074bfffffffe')
2013-07-08T00:39:53.339+01:00 [7F5ED952B700 warning 'RemoteDisk'] longContentId mismatch: Expected '' Actual '55fe6edc3dabc41e7034074bfffffffe' (path=/vmfs/volumes/51c47e89-58253ce9-1a7b-00221968514a/SVR-SQL2/SVR-SQL2_2.vmdk) (disk=RDID-e85ec2f7-98f2-4d1b-a654-e841026d0d0e)
2013-07-08T00:39:53.339+01:00 [7F5ED952B700 verbose 'Db'] SQL code: SELECT * FROM GroupInstance WHERE (groupID == 'GID-558eb100-f9b1-4d2d-89ac-ae017aceef67') AND (completionTime NOT NULL) AND (isPruned == 0);
2013-07-08T00:39:53.339+01:00 [7F5ED952B700 verbose 'Db'] SQL code: SELECT * FROM GroupInstance WHERE (groupID == 'GID-558eb100-f9b1-4d2d-89ac-ae017aceef67') AND (isPruned == 1);
2013-07-08T00:39:53.339+01:00 [7F5ED952B700 verbose 'Db'] SQL code: SELECT * FROM GroupInstance WHERE (groupID == 'GID-558eb100-f9b1-4d2d-89ac-ae017aceef67') AND (completionTime IS NULL);
2013-07-08T00:39:53.339+01:00 [7F5ED952B700 verbose 'Delta'] Prune check group GID-558eb100-f9b1-4d2d-89ac-ae017aceef67 (has 0 consistent instances)
2013-07-08T00:39:53.339+01:00 [7F5ED952B700 verbose 'Db'] SQL code: UPDATE OR ROLLBACK ReplicationGroup SET sessionID = 30 WHERE (groupID == 'GID-558eb100-f9b1-4d2d-89ac-ae017aceef67');
2013-07-08T00:39:53.351+01:00 [7F5ED952B700 info 'Misc'] SessionID 30 (token 59) connected to GID-558eb