size can be a feature that effects where it goes in the cab. other than that, you can get the simple, cheap 40 channel ones like the Cobra 19, where all you want is volume, gain (pulls in more signal & noise)) & squelch (blanks out static & faint transmissions). Some larger ones have a simple meeter on them, and some have NOAA weather channels. If all you want it for is runs, just get a $50 CB and a magnetic mount (see my ebay listing now that I upgraded
). If you plan to wheel where a flag is needed, you may want to consider a heftier antenna and maybe mount it on a piece of angle iron on the spare tire rack (the iron or aluminum goes in between the tire and the gate and you mount the antenna on the drivers side). You'll have to tune your antenna though, and the SWR meter is $20 (although you could then sell it on ebay after your 5 minutes of using it).
I'd recommend contacting a CB shop and have them help you. Walcott CB is my favorite online shop, but others can help too. Just explain what you want to do, and tell them that you just want one for X times a year when you off road, so you don't want high end equipment.