There are two measurements with regards to rivets. The first is the diameter of the rivet. The majority of rivets used on pinball machines are 1/8" (shaft diameter). Specifically, the rivet type is called an oval head semi-tubular rivet. The second measurement is the length of the rivet. The thickness of what is being riveted determines the length of the rivet to be used. Lastly, as is the case with flaps, whenever you are going to rivet where the bottom of the rivet would come into contact with plastic, you must use a washer. The thickness of this washer must also be taken into consideration when determining the length of the rivet needed. Ramps vary in thickness, but I would guess that 7/32" or 1/4" would be what you need.
Don't know if PinRestore is still selling them or not, but they have a great rivet starter kit for $30...
http://www.pinrestore.com/Riveting.html