Hi Christoper Uy,
Sizing the exp. valve you have to know the following.
a) max evaporator load
b) evaporating temp.
c) condensing temp.
d) Liquid subcooling

Then for easy selection find the catalog/book of any refrigeration components manufacturer like danfoss or alco then look on their selection table.

for your continue learning in refrigeration you can visit www.danfoss.com;)

Regards