While doing some shopping I found the most general size, including the rentals, is about 24" wide and 43 - 47 Inches long and weighs well over 100 pounds.
The "Go-Go", which is normally not a rental is only 21" wide and 39" long, and weighs under 100 pounds.
The Walker rentals are both the "standard" (larger) size for both the regular and heavy duty.