The size of the tank will not affect the turtle's size (unless it's so small that it touches his shell), but poor water quality can cause him to be less-than-healthy, which WILL stunt his growth. Do you have a filter (a good strong filter)? A heat lamp? A UVA/UVB light? A water heater? If you don't have these things, he will not live out his full lifespan, but will die of disease at some point----these are all necessities for good turtle health. He'll also need some veggies in addition to the Repto-Min, only new hatchlings (under 6 months old) can live off of Repto-Min exclusively. Here's a care sheet---if you want your turt to live a long and healthy life, you'll need to follow it closely:
http://www.austinsturtlepage.com/Car...rn_painted.htm
Fortunately Painted Turtles don't get as big as Sliders, so you might be able to get away with a 75g tank for an adult.