My most often used ruler is my 6 1/2 *24. (Just like IceBlossom) It spans the wof easily and would make short work if cutting.
I also suggest you get a square up ruler also. My personal favorite is the Creative Grids 4in1.
You may also want to look up some alternate methods of making HSTs other than the 2 at a time method. There are others that are faster - my favorite is the "strip" method, but you need the CG ruler! You can also make perfect ones on the bias once you know The tube technique, just have a care while ironing.