Mine is directly under the ash tray in both my rigs. There's just barely enough room there to mount a full-sized CB radio (like a Cobra 148-GTL, Galaxy DX949, DX99V, etc.) and have it clear both the ash tray, and the 4WD shift lever if you make your own bracket(s) that put the radio directly up against the bottom of the dash (it clears the shifter by about 3/8", though YMMV if your truck's body bushings are shot. A smaller radio such as a Uniden PRO-520XL however should fit no matter what).
Since the "all-in-the-microphone" type radios were mentioned, I would strongly suggest that if you decide to go that route, get the Midland 75-822 (I believe was the model number). DO NOT buy the Cobra version (75-NWST or whatever it is), the Cobra unit has about the worst-sounding microphone of any kind of radio I've ever heard, and because it's an all-in-one unit, you can't replace it with something aftermarket like you can on a traditional radio (not in a practical sense anyway... If you're good with a soldering iron and can do microsurgery, then maybe... But the Midland doesn't just have a better-sounding mic, it's all around a much better & more-versatile radio).