I would say definitely less than a 5.25" drive. One more point to make:
After the SE came out, many users found that their FDD would accumulate dust and gunk causing drive problems. This was due to the fact that the fan draws air directly through the floppy slots. So much so, that Apple eventually offered an optional plastic dust shield for environments where this was a problem (ut's a problem in all environments but do you think Apple was going to admit that?). The shield completely surrounded the top and sides of drive virtually eliminating air-draw through it. It would therefore also cause some heat build-up, but clearly not enough to be of concern (so it is unlikely to significantly increase the heat inside the case either). The Classic eliminated the problem by creating an air scoop which funneled the air directly up from the bottom of the case, and added the vent to the top handle, thereby reducing the draw through the floppy. Later drives of course eventually added a flap to limit the air intake.
Something to keep in mind when you install your new fan.