I thought they stayed relatively small... But mine are 2-21/2".
On a site note: my local aquarium store "Nature Aquarium" in Santa Monica, told me not to feed them to keep them algae hungry. So along with the algae, they decimate my Val, Downoi, Fissidens, and Starougyn.
Greedy little buggers... I have about 30 in my 55, purchased as 1" little dudes and the run the tank. I have 5 in my RCS tank and they are very boisterous. I have tried to remove them from my Dutch setup... Impossible so far... I will continue to try and pull them and replace them with yellow and reds...
If I did not have experience with other shrimp species i think are a calmer less destructive choice... I would still love them.