Our first TT was 20' with no slide. Lasted just one season before we realized it was waaay too small and cramped. Upgraded to a 29 footer with 1 slide with sofa in it and rear kitchen that creates lots of circulation and counter space.
Huge improvement. We now have plenty of space for the 2 of us. We also upgraded to a 3/4 ton TV at the same time and are glad we did. Not much difference in towing or backing up a 20' vs 29'.
If you indeed do find that a smaller TT isn't working for you, you can lose a LOT of $$ when you go to sell it or trade it in. Better to make the right financial choice from the start.
Some CGs have rental TTs on their sites. Maybe spending a weekend in one will help decide. If you have kids, quite a few manufacturers make a TT with a layout similar to this:
