Hard-codes the size for snapshots and movie recording. By default, MESS will create snapshots at the system's current resolution in raw pixels, and will create movies at the system's starting resolution in raw pixels. If you specify this option, then MESS will create both snapshots and movies at the size specified, and will bilinear filter the result. Note that this size does not automatically rotate if the system is vertically oriented.

The default is auto (native resolution in raw pixels).

mess -snapsize <width>x<height>

mess -snapsize auto