The only ramp I have is 16 inches wide and measured out at an angle that makes it not so steep, we got a tape measure and held it from the pop door where the nest boxes are to the floor about 4 feet so the incline is more gradual. Also have 1X1 strips nailed across the ramp, spaced every 4 inches... for traction so that if for any reason they started slipping, they could get a hold on the strips. I had a 12 inch wide one first but found that they fought over who would go up or down more and always one got pushed off.
My 2nd coop has concrete blocks for steps up, and then the 3rd coop has a ladder with rungs...16 inches wide.
I like the concrete blocks the best because they stay put and seem to stay cleaner.
My ramp is a 2X6, Its about 4'long and ends on a cement block (it saves me about three feet of ramp length). From the door it drops from 30" down to 16". I also use wood strips for traction. The girls like to perch on it throughout the day, and I hang a waterer under it (easy to access and easy to keep clean)
My ramp is one foot wide. It's the same width as the coop door so whether or not more than one chicken can be on the ramp at the same time doesn't matter as their fat butts won't be going through the door in pairs
It is about a 35 degree angle. I originally had slats about 4 inches apart but I watched them sliding on the wood between slats. I didn't want pulled hamstrings (or whatever they might get) so I added another slat between the existing ones and they love it.