Dimensions vary.
I currently use Corsair MM200 XL (about 44×38 cm, and reasonably thick), which is wide enough for a 75% keyboard and a trackball, or a 90% keyboard; it isn't square either, although I'd prefer something with a larger width:depth ratio—closer to Roccat Sense (40×28 cm), but that one's rather thin.
The large Kingston HyperX Fury is actually 50 cm wide, i.e., big enough for a full-size Model M, but I don't quite understand, why they insist on roughly 5:4 ratio, when most screens are 16:9 or even wider.
The overlap has at least one upside though. I haven't had any problems with peeling cloth on edges, even though these mats don't have stitches edges. Actually, I'm not even sure, if I wouldn't hate stitched edges on a smaller mat.
I'll probably get the C-Tech Anathea anyway; it's only €7 in a local store IIRC.