I'll pass on advice that was passed onto me: try before you buy. Handling the gun type you're curious about for a bit is the best way to find out if it's comfortable or not. And this doesn't mean Walmart clearsoft; it means finding someone or somewhere that will let you handle a quality airsoft gun. Then you'll be able to decide whether an M4 is "for you" or not.
To my knowledge, most major airsoft brands make their weapons close or exactly to scale. Some may be closer than others, but it should certainly give you a feel for the gun, either way.
Always expect the worst. That way, you're never disappointed.