I don't bother with any re-sizing for the iPad. With a fairly large hard drive I keep my photos in jpeg format usually still in mbs. Photos transferred to the iPad through iTunes are optimised for the iPad which, as I understand it, is a form of compression reducing the photos to a suitable size for the iPad. Related is the iPod Photo Cache on your pc Apple support which keeps the optimised images sent to the iPad. You will see that the thumbnails are quite small in size.
I suggest that the answer to your question is to let iTunes do its thing and not shrink before transfer as you could get further shrinking with loss of resolution.
BTW if you transfer through Dropbox then this optimisation doesn't take place.