Dr. Darren Naish posted a Tetrapod Zoology blog on the vesper bats (Vespertilionidae), which make up some 410 of the 1,110 known species of bats. (New species are still cropping up.) Anyway, that led to an interesting series of expert comments on the question of why bats are not bigger than they are. Some of the "flying fox" fruit-eaters may have wingspans of 1.8m, and there have always been claims, though without supporting evidence, of larger ones. Naturalists Ivan Sanderson and Gerald Russell, along with two African helpers, saw a specimen twice in one day that they estimated was 4m. Is that practical?
These comments postulate a number of reasons, including thermoregulation, nutrition, and the physics of echolocation, why bigger bats than 1.8m might not work, although it seems to me there is room for argument. I don't expect we'll ever see a plane-sized bat, though. As I commented on Naish's thread, "There has to be a structural limit to each type of animal wing just like there is to each type of airplane wing (over a certain size, you have to add more spars, different construction methods, stronger and lighter materials, etc., and this is easier to solve with fixed wings than with flapping wings). If pterosaurs could get to 10m+ with a wing that seemed less well supported and braced than that of bats, though, I always wondered why bats didn't get bigger."