Using Excel VBA to use If statement to pick value from a list

In a financial services setting, I have accounts that each have a 2 digit fileset ID. I have a list of filesets broken into 3 groups: Small, Medium, and Large.

I want to paste a list of accounts with their corresponding 2 digit fileset. Using the fileset in an IF Statement I want to click a button to put the group in the same row as the fileset.

I cant seem to get it to work even using a loop and a series of complicated IF Statements. There has got to be an easier way to do this.