The size thing is a non issue really. Hell, in Mario 64, Sunshine and the Galaxy games, Bowser is WAY bigger than Mario.
Ridley's exclusion is mainly because Metroid is not something on Sakurai's radar especially as it's more popular in the West than in Japan. Unfortunately, he gave us 3 Kid Icarus characters instead, 1 being a clone.
Also, you know how before matches a little 'tip' comes onto the screen? Apparently many of those actually explain advanced techniques, such as Smash DI and pivot grabbing. Pretty cool addition