Actually not a car, but a pickup with a rack.
No canoes or kayaks for me, too unstable.
Something around 10' - 12'; to float two fisherman, a cooler of beer and an electric trolling motor.
A lightweight Jonboat is the lightest and handiest for what you describe. I wouldn't go less than 12 feet in one of these. I've had the "pleasure" of fishing out of a 10 footer with another adult and it was okay as long as neither of us moved! I think we had about 1" of freeboard all the way around!
With 2 guys to take it off and on the rack a 12 ft V-bottom that's not real deep and wide (read that "heavy) will work good. We used those for years when fishing drive in lakes up in Canada. The advantage is they are much more stable than a typical narrow lightweight Jonboat.
If you plan on doing some of your fishing alone then a light 12 foot Jonboat is probably about the best compromise for ease of use.
I have a 15' Carolina Skiff with a 30 Etec on back for the river nearby, but I picked up a Bass Pro Topper 12' (actually 11'10" if you water hole measures) and it does very well for the wife and I in the local ponds. The Bass Pro is rated for 5.0 HP, but a similar jonboat at Cabelas has a 9.9HP rating.