Most used is the 6.5 x 24 size, then my 6.5 square and my 12.5 square. I always use the 6.5 for squaring up HST. Just line my fab up so there is a little excess to the size I want trim trim, then flip the unit and true line it up and trim the other two sides. Can't see much reason to buy a smaller ruler when I can simply flip the unit.
Re self healing mats.... I swear by my Olfa. I started quilting almost 20 years ago and bought an ofla then. It would still be going strong except I stupidly set my hot iron on it. So I replaced it with a fiskars one thinking same thing. Not so. I bought another Olfa last year when JoAnnes was having a 50% off sale on all olfa products. I think they truly are self healing.