Here's the basics for road pedals:
Cleat formats (none of which are truly interchangable):
Look (classic - 3-bolt)
Look KEO (3-bolt)
Time (old 4-bolt)
Time (new 3-bolt - Look bolt spacing)
Egg Beaters/Quattro (2-bolt)
SPD-SL (3-bolt - Look bolt spacing)
And there are others - those are the heavy-hitters. If you had to boil it down, the popular ones are Look (both classic and KEO), Speedplay (Zero and X), SPD and SPD-SL.
Cross-compatibility between the systems is fairly slim. You can get generic Look (classic) and SPD pedals from many manufacturers (e.g. Wellgo, Nashbar/Performance, Ritchey), and they work with the official cleats. But the newer standards are still covered by exclusive patents.
If you want road stuff that's cross-compatible with MTB, your choices are SPD and Egg Beaters. The biggest gripe most have with using a MTB cleat design on the road is the tiny contact patch between cleat and pedal, which can cause hot spots if not dialed in just right. Also, the MTB cleats (save for the Quattro version of the Egg Beater) don't really stand up well to full-tilt sprinting, as they're designed to release more easily and in more vectors.
As far as shoe size is concerned: one man's 44 is another man's 45 is another man's 44.5 is another man's 43.5. Try on lots of shoes. If you have a brand of MTB shoe that fits well, it's worth trying the road shoes from the same manufacturer.
Note that most high-end road shoes are exclusively 3-hole drilled, and won't accept 2-hole cleat systems (e.g. SPD or Egg Beaters) without an adapter, which adds to sole thickness and can affect bike fit.
Hope this helps!