Looking at a femal from behind - at the closest visible point to her vagina - you will see a gap between her inner thighs. Draw an imaginary horizontal line along the gap and draw one along each thigh. If you connect all lines, you'll end up with a trapezoid shape(trapeeze). The bigger the gap, the bigger the trapeeze. Trapeezes are messured in male fingers. In order to determine trapeeze, just guess and / or try how many of your fingers would fit the gap.
Rolls: Yo dude check her out, she has a huge gap between her thighs!
Royce: Yeah man, that's at lease a two finger trapeeze! I'd totally do her!