The inside measurements of my rv garage are 18x40. Our 36 foot fifth wheel fits perfectly with plenty of room to walk around the front of it. Also, there is room enough to open all 3 slides as long as I am centered. I did spray paint a stripe on the concrete floor that I follow that let's me park it correctly every time. I do wish it was wider and longer but it's all I could manage. Mine is enclosed on all sides with a 13 foot roll up door which I also wish was a little higher. I agree you will need more length to protect the front and rear if the front and rear are open.
We are getting ready to start our 5th season with ours and generally use it around a total of 3 months. It still looks new and outside maintenance is greatly reduced if you can swing the covered parking.
Another consideration is if you can get some hook ups to it. I have a 50 amp service, water and a 4 inch sewer dump in mine which comes in mighty handy.
JimT
2020 Jayco Pinnacle 32RLTS, 2020 Ford F350, Platinum, 6.7 diesel, 4X4, CCLB, SRW, 12,400 GVWR