I bought the 2.5" and 4.5" block loc HST rulers. I figure I'll never use the 6.5" as that seems huge to me. And if I do, I'll just use my regular rulers to trim. These are something I wish I'd bought a few years ago!
I really love the 2.5" the most, but I am into tiny.
I really want to get the JT trimmer tools, but they are so big! I can't imagine trying to use a 5"x10" ruler to trim down a FG unit that measures 1" on the short side. I've considered buying it and cutting it in half. Right now I usually use my 3.5" ruler and know where to line it up for trimming.
My most used ruler for trimming is my 3.5" ruler. I have rulers in 1" increments up to 12.5" and then have a 16.5 and 24.5 squares.