I use the 24x36 Olfa mat on the un-gridded side. I have my old whitish-grey Fiskars mat underneath it with enough of the measurements showing that I can quickly estimate with it.
A lot of folks complain about the chemical odor that they detect (China's factories are notorious for that) when shipped in a box and sat for awhile. For that reason, I bought mine at JA, chose one from the back of the rack , figuring it had been there airing out the longest. Gave it a sniff test and wasn't overpowered by it. It's been fine in my house.