Hi. You asked, how much room? For 2 people, 24 feet long would work OK. If you need more room, 26 to 28 feet would provide ample room. I would avoid anything longer unless you plan on taking many things with you on the trip. Smaller RVs will more easily park in remote campgrounds, and are easier to drive through cities and towns. Parking and gassing the RV is also a bit easier in a shorter RV.
Which brand of chassis Ford or Chevy might depend on some of your physical attributes. Chevy is reported to be better for tall or big individuals because the cab area is roomier. The Chevy is reported to get better gas mileage than the Ford. The Ford has a very powerful and durable engine which allows it to climb hills without much difficulty. Both Ford and Chevy produce very reliable chassis, so you shouldn't go wrong with either with lower miles on the odometer.
Unless you have a strong reason for choosing an RV based on the chassis, I would look more closely at the construction and floor plan of the living space. I would suggest avoiding a rubber roof. Make sure the sleeping area(s) will meet your needs (size and layout), and check the bathroom and the shower. Make sure the generator runs well (if equipped), and all appliances work as they should.
There is much more you can learn by reading some of the postings on this forum.
Good luck with your purchase and let us know when you are coming.