Large 21" shows a top tube of 23.5". I presume that's
effective top tube length, as if the top tube was a traditional level/horizontal top tube.
http://www.giant-bicycles.com/en-us/...8882/#geometry
Wearing bike shorts and in stocking feet, hold a length of something straight, like short piece of PVC tubing, pulled up snug to your crotch.
Have someone measure the distance from the floor to the top of that level tube.
Take that distance, multiply by 1.09.
Take that distance and transfer it to the bike:
Parallel the crank arm to the seat tube as in the website picture.
Level the crank arm horizontally.
Raise the seatpost and seat to that distance, measured from the top of the pedal level to the top of the seat.
This is an approximate rule of thumb to get roughly the best seat height.
Get some sort of plumb bob, like a small nut on the end of piece of string.
Sit on the bike in stocking feet. Level the crank horizontal. Place the ball of your foot on the pedal spindle.
Place the string on the front of your knee below the kneecap. You want the plumb line to line up with the pedal spindle.
Slide the seat, level, forward or backward till you get that.
If the numbness is still there after riding, you need a different height/length stem or a different size bike.
Fwiw.