I haven't really given it that much thought even though I do eat at Taco Bell at times. I guess as I get older I don't eat as much so don't really order the larger sides but you are right about the absence of multiple sizes. One thing about Taco Bell that I do like is that when they tend to do away with a menu item they leave it on the menu and you can still order it. For instance the MexiMelt or Encherito (if I spelled that right).
Another thing I've noticed recently is that the new location, in front of the Hampton Inn Suites seems to be better, fresher, tasting than the other locations but then it could just be all in my head because it is the same food. I do hate, though, that the sweet tea isn't as good as the other locations. Thinking about it I do think they could easily offer multiple sizes of their sides and given that, like most fast food restaurants now, they utilize monitor screens it wouldn't pose much of a problem as far as the menus go. If I get a chance I'll try and put that comment in next time I fill out a comment to Taco Bell. Never know could help if enough people made those type comments.