I have an L1200 too.

The specification says max print size on baseboard is 19 3/4 x 27 1/2 in

I was lucky enough to get hold of a wallmount for this enlarger and that allows me to use a beard 20x24 easel which gives a max image size of 20x16 with 2in borders.

I doubt very much that an M601 enlarger would allow more than 20x16 paper on the baseboard if that.