That's really a personal decision based on budget, use, space, etc. I have a 14.5" and a 22.5". My 14.5" is for the RV and the 22.5" is a permanent fixture on the Big Deck. I've done a pork butt, a whole chicken, two racks of ribs (cut each rack in half), a spiral sliced ham, fattie, etc, etc on the 14.5" and have only used the top food grate. I imagine I could do more if I used the bottom food grate. Of course, the 22.5" can cook anything you wanna put on it but I imagine the largest thing I've cooked is a HUGE 22+ lb turkey.
Think about what and how you wanna use it and go for it. HTH.