check out this thread...
https://www-secure.symantec.com/connect/forums/seamless-veritas-volume-migration
To answer your questions:
Do we need to allocate the disk manually to different volmes.?
-- It is your choice, if you don't allocate manually, vxvm will pick disks on its own & mirror.... I would prefer to allocate here since we need new plexes to explicitly go on new disks from new storage...
Do we need to have disk of same size to be added into Diskgroup.?
-- again your choice, disks can be equal or bigger in size, they can't be smaller... for e.g if your current volume is a concat volume of 100GB made by 3 disks, you can get 1 single disk allocated by storage of 100GB & mirror with this...
Secondly,I need to ask storage team to provide storage for data migration so how to calculate how much disk(LUNS) required and what should be their size to migrate the data from every volumes.
-- sum up size of all the volumes & you should get the current total usage.... additionally you can keep some extra storage available for scalability.... commands that may help here:
# df -h or df -k
# vxprint -qthg <diskgroup> (check the dm entries for each disk size)
# format (you can check the size of s2 on each disk)
Gaurav