If you're using export in QT Pro, there are options including resizing that you should check... they're there in the save as dialogue. If you're using simple save as, it shouldn't alter anything.
In Quicktime, hit command-I (Mac) or ctrl - i (PC) and an info window comes up and shows you details about your file.
cmd / ctrl - j gives more information on a per media layer basis.
After Effects is also pretty good for examining files as they actually are. i.e. without colour management, at native pixel size and frame rate etc. But as you point out, AE doesn't much like mp4.
Be warned though... although Quicktime gives you a bit more info it does sometimes lie about actual pixel dimensions... giving the display dimensions as actual pixel dimensions. Seems to happen with mpeg2 stuff exported from DVD studio, for instance. No idea why this should be..... a bit like Quicktime's famous gamma problem... utterly mysterious.