I don't do a lot of woodworking, but I have to do some for work in one of my homes. This is a good topic for rec.woodworking
I've had the same complaint about too-small miter boxes, I can't fathom why they can't even hold a normal sized piece of baseboard. My solution was to buy an electric miter saw from my neighbor, used for $30.. if you check the prices in stores on 12 inch blade miter saws, I think I got a darn good deal. I did, of course, have to buy a new blade. The plastic safety shield was broken, so I ripped it the rest of the way off (those really get in the way).
Pic linked of my saw. Please don't mind the filthy basement. I don't think the laser guide works, but you can set the angle accurate enough to do nice 45's or whatever. I would not get a smaller blade miter saw, considering that you'd be in the same boat of not being able to chop trim; this time because the blade won't reach.