So, hearing so many people saying flat out that 18' boat is way too big for a family of 6 in Colorado has made me wonder: What multi-day trips are you guys considering that would be appropriate for 4 kids under age 5? I can only think of a few in Colorado and surrounding states that I'd take my almost 2 year old: Upper C, Ruby/Horsethief, San Juan, Labyrinth/Stillwater, maybe something on the Upper Bighorn Sheep Canyon on the Ark. With the exception of the Ark, all those are plenty big enough for an 18' boat. And even then, I'd be plenty happy with my boat on the Ark at anything above about 800-900 cfs.
Keep in mind: bigger boats draw less water for a given load. That means that as water drops, big boats remain more runnable than a smaller boat given the same gear, until the point that you have a narrow pinch-point.
To be clear, there are plenty of smaller streams that won't handle an 18' boat well, however to say that an 18' boat isn't appropriate for anything except the Grand Canyon is a gross exaggeration.
This discussion has also made me wonder how many of those who are recommending a small boat to the OP have kids and wives. I know that but for the equipment "needs" of a wife and toddler, I could take care of myself for 3+ weeks on a 13' boat. That space budget goes right out the window once my wife starts loading the car.
Lots of good advice here, though, and hopefully a good demonstration that there are so many factors that drive what boat or boats are best for people.
Last point I'll emphasize, that was made once on this thread, is that thanks to folks claiming that 18' boats are unrunnable, the prices on used 18' boats can be pretty low compared to similarly abused 14'-16' boats.