Well, first lets determine what won't work. CAE
(chinese algae eaters) and common plecos get way too big. Oto's don't eat a lot of algae. Your best options would be the dwarf plecos like bristlenose or pitbull. There are lots of other small plecos, but those are the most commonly available (and cheapest). SAE
(siamese algae eaters) get up to 6" long, but are the most versatile algae eaters. They are one the few that eat many varieties of hair algae. Their main drawback is that it can be hard to find true sae
and not get stuck with a worthless flying fox instead (prepares for hate-mail from flying-fox lovers).