I don't think you should move him solely because of his size. If he can provide league-average defense, he is far more valuable to our future at 3b. If/When Morneau leaves or is traded, Mauer/Doumit/Parmelee (maybe even Plouffe) should be able to hold down 1B for at least the next 5 years.
Plus, if you work like hell on his defense at third, and the conditioning needed to play 3b every day, I think it will be better for his long term career (in terms of health and baseball IQ) than just shuffling him out to catch flyballs or standing on 1b waiting for balls to get thrown at him. You can always move him later.
Don't forget, Gentleman Jim was exclusively a 3B for the first five years of his major league career, and I think we could do worse than Sano turning into the next Thome.

