Find the MAX value of changing sets

I've been fiddling with how to do this for a coupe of hours, to no avail. Time to ask the experts!

Given three columns (SET, SAMPLE, and SIZE), I'd like a formula in a fourth column that give the maximum value of SIZE for all rows that match both SET and SAMPLE. There's got to be a simple way to do this, but my brain just isn't functioning at 100% today.

My only real restriction is that it must be a formula, not VBA or anything.

Example datasheet attached.