I usually don't recommend any type of algae eating fish for any tank under 10 gallons, and 10 is pushing it.
Trinidad or common Plecostomus: Can get to 2 feet if the water is clean and they get enough to eat. Often die of starvation (You can tell they're starving when they start trying to eat other fish's slime coats.) Recommended tank size: 125 gallons+ or pond
Chinese Algae Eater: Not a true algae eater. More closely related to the flying fox. Parasitic/predatory fish. Reaches 6 to 8 inches in length. They eat algae as juveniles, but as adults they turn predatious and attack other fish for their nutritious slime coats (Not a sign of starvation in these fish, it's natural for them to do this.) Not recommended for community tanks.
Siamese Algae Eater: True algae eater. Can be anywhere from 3 to 6 inches in length as an adult. High metabolism, needs lots of food, so supplemental wafers are a must with this fish. Recommended tank size: 40 gallons+
Otocinclus: True algae eater. Maximum size for a large female is 2 inches. Most will only reach 1 1/2 inches. Extremely high metabolism! Survives best when kept in a school of 5 or more, solitary fish tend to live for only a short time. Driftwood in the tank will help them survive better as they need the wood fibers in their diet. Most of these sold in pet shops are wild caught because they are extremely difficult to breed in captivity. Recommended tank size for a single fish: 10 gallons. Add 5-10 gallons for each additional fish. Supplements will be required.
Bristlenose/bushynose Plecostomus: Max size is 6 inches with most being around 4. Relatively easy to care for once they have adapted to your tank (takes about 2 weeks). Hard to find in some areas. Recommended tank size: 20 gallons+ though they can live in a 10 gallon if they're supplemented and have driftwood to chew on.
Clown Plecostomus: Adult size for the females is 3 inches with males being around 2 to 2 1/2. Also easy to care for once they've adapted, but even more difficult to find than bristlenoses. Recommended tank size: 10 gallons +. They can be kept in a tank as small as 5 gallons, but, again, supplements are a must along with driftwood.
For smaller tanks, I will usually recommend a snail or a shrimp as the algae cleaner. Much less impact on your bioload and they can eat fallen food as well as algae.
If anyone has other info or corrections, please feel free to add in. If I took this off topic, I apologize, just wanted to get the info out there.