How do I figure what height I need for my egress window wall?
The minimum height of the window is specified in code, and so is fairly standard.  And is not typically influenced by the well dimensions.

The bottom of the window must be within a specific height above the basement floor.  The well has to go from that depth, to at least finished grade level.  So that's what determines the size of the well, not the window.  

The code also states that if a well is deeper than a certain depth, a ladder or stairs must be included (to get out of the hole).  So I would assume the manufacturer made the well less than that height purposefully, so as not to have to include a ladder.

Measure from the basement floor, to the maximum code specified height of the bottom of the window. Then from there, to grade level.  If the second measurement is greater than 46", you'll need a deeper well.