20x20 is very small, I have an attached 19x24 on my house with a 17' door, fitting my Elantra and our Grand Caravan in there is very tight, I have to back in so we both exit on the inside of the garage otherwise there is no room to open the doors. You can just barely pull the van far enough ahead to open the rear hatch but it will still hit the door just before it gets all the way out. It's enough to get things out of there but it is really cramped for sure.
Go oversize, as big as you possibly can, seriously.