Cot Size Bed Dimensions are all over the place. The major sizes are 28" x 77", 25" x 77", 32" x 82", 40" x 85", 30" x 75", 30" x 72" and 34" x 72". These are Cot bed measurements. Cot Mattress depths can vary from just an inch to a few inches to 6 inches or more Many times, actual cot mattress dimensions will vary by one or two inches, usually smaller.