What is the maximum length and width a ramp can be that meets ADA requirements ?

There are no maximum length and width requirements for ramps. There are maximum widths for handrails and maximum rises for ramps before a level landing is required.
405.2 Slope. Ramp runs shall have a running slope not steeper than 1:12.
405.5 Clear Width. The clear width of a ramp run and, where handrails are provided, the clear width between handrails shall be 36 inches (915 mm) minimum.
405.6 Rise. The rise for any ramp run shall be 30 inches (760 mm) maximum.
405.7 Landings. Ramps shall have landings at the top and the bottom of each ramp run. Landings shall comply with 405.7.
405.7.1 Slope. Landings shall comply with 302. Changes in level are not permitted.
EXCEPTION: Slopes not steeper than 1:48 shall be permitted.
405.7.2 Width. The landing clear width shall be at least as wide as the widest ramp run leading to the landing.
405.7.3 Length. The landing clear length shall be 60 inches (1525 mm) long minimum.
405.7.4 Change in Direction. Ramps that change direction between runs at landings shall have a clear landing 60 inches (1525 mm) minimum by 60 inches (1525 mm) minimum.