I don't necessarily repot at all. If what I bought is a 1 gal. annual, I may just sink it in a really big pot with other stuff and then toss it when its finished, keeping the pot, of course. If its a perennial then I want it to be in a pot it can stay in. I've never yet had a problem planting a perennial in a much larger pot than the nursery pot it came in. I usually surround it with annuals until its full grown and then decide what to do next. The politically correct rule seems to be to pot up the plant by no more than one or two inches in circumference greater than the pot its in. That just seems like too much trouble for me, but if a plant doesn't make it here its just gone and I cheerfully plant something else. You may not be as casual as I am, so the more conservative advice may do you better.
