I just finished my carport this week. Went with 20x40 size, 11 foot legs. The center height worked out to be 14 feet high. If I park the Vengeance 25V (8.5' wide x 31' long) slightly toward the left side there is plenty of room to put a car on the other side (probably two cars). I don't have a slideout so can't tell you if a car would fit with the slideout extended. However, even with the car parked next to the RV I can still extend the awning (to dry it out).
I ran an underground line to hook up to a campground style 50/30/20 amp power box. I only needed 30 amp but just in case I upgrade to a 50 amp RV I'll have the power available. It only cost about $40 more for the three way as opposed to the two way. I also ran a water line to the carport so I could hook up a hose if needed.
I decided on the 40 foot unit rather than the minimum 35 foot needed in case of a future RV upgrade. The side posts are on five foot centers so you can get lengths of 20, 25, 30 etc etc. Increasing the length from 35 to 40 feet cost $300 and the metal roof panels are un-seamed 40 foot sections.
The cost for the building was $2,900 installed. Cost to run the electricity (with outlets) and water was about $400. Don't forget the "base" material. I used 5-7 size gravel and the cost was $310. A single truckload of gravel is adequate for the carport and would probably cost more but I got three loads so I could re-cover the existing driveway and parking area so there was a slight discount. Would have preferred concrete but that would have doubled the cost of the project..or more.
Except for the actual building set up I did all the work myself. I'm a little slow running the tractor/box blade setup for spreading the gravel so the entire project took three days (the building was completed in seven hours).
Here's a few pix: