I'd also just like to add a comment about fuel costs as it is an important one that was brought up. Having towed both the Bambi and the Safari, the gas mileage didn't change at all, it was equally dismal at about 10.5 to 11mpg for either coach. I'm not sure either way if the weight difference (gross) of about 1400lbs made any difference at all. I think it's just wind and the weight in general. Now clearly if I had a 34' unit or a unit that had a weight difference of a few thousand pounds, then perhaps a bigger difference might be realized. In my case though, there was no difference in MPG from what I saw towing our 19' Bambi compared to our 25' Safari. I will admit that at and over $2 a gallon any way you cut it, it's not gonna be pretty.
As for limitiations in places to go based on unit size, it's is for sure gonna happen. So far though, including going RVing as a kid with our 24' Jayco, having been to nearly 40 of the 50 states (and 4 provs in Canada), we have never come into any issues so far...that doesn't mean that they are not out there. I would think that the larger you go, the harder it gets, but again, for every one that can't take larger than a 19', there are 1 or 2 that can, maybe more.