757Mustang

I was detailing my engine bay and i noticed my idler pulley from the previous owner, was loose like i can wiggle it. so i want to replace it so what s/c pulley should i do? i thought about the 2.80 size. I do know i have to have a tune which i got covered but i just wanted to know what size is good. i have a 3 inch i believe from the previous owner. also do i need a different size belt???? The other problem is what size idler pulley should i get. Steeda has a 100mm one? will that work? Thanks for the help guys