Now that I think of it, would it not be simpler to just add a upload link when a large is deemed "missing"?

I think the problem with the workaround of changing the config to the small size is that 1) IM complains it's less than that of the admin setting(dunno if that is actually a factor) and 2) if the image size is different in height or in width you can't accommodate that. ie I think in many cases product images are not square. In my case almost none are...Since IM doesn't like a setting of 0 for height or width it will never ask to "use small" if one of the dimensions is under the value set.