Fire box size, can i build a bigger one?

I have a basic offset smoker, char griller (their competition one hahaha from homedepot) Anyway, I'm new to all this, but the fire box on this thing is pretty small. The fire lasts about 30 min before you have to tend to it, adding wood etc... (maybe this is just for charcoal, IDK, but I like the flavor of cooking with the real wood better) Can I just build a bigger fire box so I can have a fire that lasts longer? I would add a "vent" between the fire box and the cooking chamber to help control the heat, but I just think a bigger fire would last a little longer than 30 min... thoughts?