This is going to spark some controversy, but it is my opinion.
I would only take up "live" self defense arts. Arts practiced against a resisting opponent with as little rules as possible with the proper protective equpiment. That being said, these are the most practical arts.
Boxing
Wrestling (I love greco as well as freestyle)
Judo
Jiujitsu (BJJ in particular)
Sambo
Muay Thai
Kickboxing
Kyosukin Karate
Kali
Col Jeff Coopers "Gun Fu"
All of these are practiced full contact, all of them have their particular range that they are good at. Cardio and neuromuscular endurance aid in these activities as well as flexibility. Size only matters if everything else is equal, then bigger is best. Have fun, it is a lifelong journey. And no one art has all the answers.