Don't forget www.petshrimp.com ...probably the best shrimp resource I have found. Also, if you are doing a hardcore plant tank, with lots for fertilizer, then most shrimp will be to sensitive for it. As far as size goes, I know some ghost shrimp can near 2-3 inches, and filter feeding shrimp (bamboo shrimp, etc) can get larger.
3-4 is fairly large for the more common dwarf shrimp. If you're looking for algae eaters, check out the Amano shrimp (Caridina japonica). They max out around 2".