I have four Fiskers mats - 1 24x36, 1 18x24, 1 13x18, and 1 13x13 revolving mat. I use them all for different things. The revolving one I use for squaring up blocks, the smallest rectangle I use for traveling to classes and retreat, the medium one I use beside my machine, and the large one I have a cutting table for.
I have four rotary cutters, three Olfa and one Fiskers. I have a 60mm that I love for cutting from yardage, a 45mm one that I use for cutting squares, another 45mm one with a retractable blade, and a 28mm one for small cuts.
I'm not a big fan of the one with the retractable blade just because the way you hold it is weird to me. Anyway, my first mat I bout was the 18x24 and the 45mm rotary cutter and that lasted me about 8 months until I decided to go with a bigger mat.