Mine are up to about 4-5" in my 75G (~2.5 years). I'd definately say 6" is probably close to thier adult size. Jigger - a sole Yoyo probably isnt at his best. Should be kept with thier own kind like just about all the loaches should. Check out http://www.loaches.com/species-index/botia-almorhae for info on the Yoyo (loaches.com)
Wow, I know thier patterns change with time but the second two dont even resemble the original 'yoyo' pattern. BTW congrats on keeping them for so long!