Refugium size is based off what you are wanting out of it. I run a trigger systems sump on my 75, and the fuge is a middle chamber fuge that is 9.5" L x 11.5"W. It does the job I need it to, as far as reducing nitrates and phosphates to a degree and is excellent for growing pods
As far as a protein skimmer, I would go +1 on the gallon rating. I run a Bubble Magus NAC7 on my 75 that is rated from 135g-185g. It does a great job and is small enough that it doesn't take up a huge amount of space in my sump.