Actually, if I were you, I wouldn't shorten my stirrups. I'd make them longer. When you ride with stirrups that are too short, your legs will tend to slide forwards when you sit up, and slide back when you lean forwards.
When you are on the ground, put your hand against the stirrup buckle. Then shorten/lengthen it so that the stirrup is against your elbow. When you mount, you should be able to comfaterably sit with your legs under your hips. If you can't, get someone to fix your stirrups so you can.