I was thinking about karate... Anyone has experience with defending yourself against bigger guys?

aikido is so good for self defense,, trust me

to add onto this, usually people are better versed in the big dojo variant, so i'd start there personally.

Ok cool thanks but will aikido help me against a bigger guy?

If you want the best chances of surviving against a bigger guy, do Brazilian jiu-jitsu, judo or wrestling (if you can find it/have the opportunity). Boxing/kickboxing is effective, but trying to knock out a bigger guy will be very risky unless you're elite. Traditional martial arts, like karate, won't help you so much outside of their own frameworks. Wouldn't really consider anything else.

Gun probably will

i would probably use a martial art like boxshu, with instagib it will be easy

I would probably say Taekkyon, because you are at a short distance and small strikes usually do best especially if you are faster.

just shoot the motherfucker lol

Honestly if we're talking about an amateur then really you don't need anything. Punch his nose and he will stagger and probably give up. Most people never been punched in the face so they get real shocked by how much damage a bare knuckle punch can do.