Yes, you can have multiple NICs for the VMKernel portgroup. The VMKernel is just connected to a vSwitch and you can add the NICs like for every other vSwitch.
In our environment here it is working fine, the NFS connection stays active even if I unplug one of the NICs.
Regarding the maximum size of the NFS I assume it is stated somewhere in the manual.
However, you should avoid to have multiple NFS storages as this might decrease the performance (I think I read this in a best-practices document)
