To measure stand up pouches you begin with the width followed by the height and bottom gusset.
The bottom fold is called the bottom gusset; this is measured when the bag is lying flat and unfilled. You take the measurement from the bottom gusset up to the top fold. 2 to 3 inches is a typical bottom gusset in a small stand up pouch, which will open up to 4 to 6 inches respectively when you fill with product.
Many stand up pouches have a re-closable closure at the opening. You need to take this lip into consideration when measuring because it is above the seal or closure and is not part of the fill-able area; you do this by measuring the distance from the opening to the closure.
A good optional feature on pouches is a tear notch at the top of the pouch; this is used for an easy open of the standup pouch.
Any questions give us a call; we will be glad to help with your sizing and design.