My first trailer was a 20" diameter and this is what I found...
1.) When doing ribs, 20" was too short for them to fit across the diameter and I had to run them long way inside the smoker. This took too much space, I thought.
2.) My water pan wouldn't fit since I use a full size pan as the water pan when I use it.
3.) The space seemed cramped when working multiple meats with basting and such.
I upgraded to a 24", which is where I am now, and it has done wonders with not only how I cook but how different meats can be arranged on the grates. There is also room for a second rack if I want. I want but have no need so I haven't done it yet.
All that being said, I am looking to upgrade to a 30" or larger or possible a vertical box smoker for the back of my trailer but am still planning out which might work best.