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An 8 inch maybe, or a very cheap 10 inch. You'd likely have to figure out a way to raise your seats to give it proper room. If you are a short guy then you shouldn't have any problems. Also, two 10's in that little cab would be /WAY/ too much sound. I can hurt my ears with just the big Pioneer 6X9's in the back.
Well I have a king cab with 2 12" subs and 2 500w amps, but I did build a box for a girl that did fit but a little tight. What you need to do is buy some speakers that require less air space as possible. you will have to tapper the box as so ----
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A 10" is probably the biggest, the girls speakers required 1sq ft of air space. You will use 1/2" wood and measure the speaker to the back of the magnet to make sure the spaeker will fit into the built box :-) have fun!