My first and only smoker is a 22", I've had it about three years, and it's perfect. There are two adults and two kids here and I almost never use the bottom rack. So yeah, in a sense I don't need that unit that large. But a) it's a lot more convenient to just use the top and not be screwing around with rolling ribs or whatever other games people use to get things to work with an 18", and b) I've had occasion (kids birthday parties) to make ribs for a couple of dozen people and suddenly that extra space was darn convenient.
I will warn you - if you start feeding friends and neighbors you will need more capacity than you think. Somehow I'm cooking big prime ribs for Christmas Eve dinner once my mother-in-law tried some I was having as leftovers. And once you do it once it's a tradition. Plus a smoked turkey, and maybe a smaller prime rib for somebody's birthday and then there's the ribs at the neighborhood block party... even if it's only occasional you may find the ability to crank out 6-10 racks of ribs without a lot of fuss to be really handy.
I don't entirely understand the urge to use a smaller ring. I always just fill the ring, close the vents at the end, and then next time stir out any ash and dump fresh lump on top. Maybe it's because briqs don't reuse as well although even there I think I would just fill it up half way instead of buying something. But if the large charcoal capacity gives you trouble, hey, it's always an option. But I'd rather just have a ton of charcoal in there just in case. My only warning with this is to not do this until you've done a couple of cooks and spattered some grease inside the interior and you have a nice seal so you're sure the fire will go out in a timely fashion.
Anyway, if you have serious storage issues or if the extra money for the 22" will be a hardship the 18" is a fine smoker and with the tips here you can get it to do almost anything. If you just want to make things easier on yourself and know it's a smoker you won't grow out of even if you get sucked into cooking more food than you thought spend a little extra and get the 22" and you won't risk wanted to upgrade later.