I have a 22 and it is huge. Lots of real estate. I was debating on an 18 or 22. I went with the 22 for many reasons. Even though a lot of my cooks will not require all that space, I can feel comfortable that when I do have a large cook, I will have plenty of space and not have to play Tetris with my meats. I figure I would do at least 1 large cookout for my son's birthdays in July where I can do like 4 racks of ribs and a large butt or brisket. Then another large cook on Thanksgiving where I will smoke at least 1 turkey and maybe 2. We have a large day after thanksgiving party where we will have about 30-40 people. And If I do the turkey(s) outside, I can have the oven reserved for other sides.
Now the only downside I can see is if I do a small cook, I would be using more fuel and more charcoal. I would rather waste charcoal which is relatively cheap than have to worry about not having enough room. And in any case, If I need to do a smaller cook, I may look into investing in a WSM 14. Now I would have to convince the wife that it is a good idea to have a large and smaller smoker....