The definitions of "big", "mid-size" & "small" firms can be confusing because I think some "big" firms are in fact "mega" firms. With that clarification, I would say mid-size is around 25-50 and I would apply to those firms now. I think firms of that size, while they may not know how much work they're going to have this summer, normally do or don't have a policy of hiring summer associates rather than deciding each year winter or spring. If that doesn't pan out, I would then apply to small firms this winter (I'm assuming you would rather work in a mid-size as opposed to a small firm.)