I'm just getting started, and I had a 6 x 24 I bought secondhand, then a couple of years ago I got Omnigrid square ones some of which came in a set. I have squares 2-1/2", 4-1/2", 6-1/2", 9-1/2", and 12-1/2". I also got a 4 x 14 because it seemed like it would be handy when I didn't need to drag out the 6 x 24. I'm cutting squares in anticipation of making my first Four Patch top.
Triangles are beginning to intrigue me as the next step and I'm eyeing the Fons & Porter ones that have markings for both half-square and quarter-square triangles. Hoping JoAnn's has them in stock at the same time I have the best coupons!
At some point, too, I'm going to need to look into getting a bigger and nicer mat. I have a mini mat which works good for cutting small pieces, and a mid sized June Taylor one that's okay but I'd like a nice Olfa or Fiskars. Finding a space to put it where I live is a challenge, though, as I share space with a roommate and some "inquisitive" kitty cats . . .