As I've said before, another disadvantage of small wheels is that the small wheel will give you less gearing *range.* This is because of the way front derailleurs work - 'normalizing' gear ratios means larger chainrings but you're stuck with the same size steps between rings. The bottom line is that if you try to 'normalize' your gears at the high end, you lose a gear or so at the low end. This doesn't apply if you leave the entire range lower. The main advantage of trikes is that you don't fall over no matter how slow you go; and with a smaller drive wheel and the lower gearing associated with it, you CAN WILL go slower. This is not exactly a selling point for me, but it is for some people.
You can only fit a large wheel if the stays are long enough to accommodate it. Within that limitation, you're not going to over-stress the stays. However, it is true that a large diameter wheel is weaker laterally than a small wheel. It has less triangulation for its size. I don't think this is a significant factor for most riding situations.