How do you calculate e-beam size from 5pA current and 40μm pattern?

[fredrick08]

Homework Statement

Does anyone know a method or calculation, that can calculate the size of an e-beam? I made a 40 square micron pattern, in approximately 30 seconds with 5pA current if this helps... I have no idea how to estimate the size of the beam tho.

 

[fredrick08]

or does anyone know the average size of e-beams?