How big? As in screen size? You can set it to be full-screen on any of the Mac monitors as far as I know. If you are talking about the size of the virtual disk format I would assume that that would be limited by the filesystem in MacOS. The current implementation of HFS+ supports files up to 16TiB. (Where 1Tib = 1024 GiB = 2^40 Bytes) FWIW: Windows NTFS file format only supports up to 2TiB in basic disk mode. Not that they make disks that big quite yet anyways. Information from here and here.
unused_user_name, one concretisation - Parallels can't operate drives larger than 128 Gb. And video resolution should be fit into 8Mb video memory.
