Mucks are mfg with membrane to form stretch up to EEE(Xtra(Triple) wide) widths. I have a wide size & they fit perfectly. They do list W size in their size chart as an option but didn't search out what boots.
Do The Boots Fit Wide Widths?
Yes. Our boots are made to standard widths, but the materials will stretch to fit widths up to a EEE. For narrow feet, we suggest wearing a thicker sock or putting in an additional sock liner to take up room.
I have tried getting different brands one size bigger for 2 heavy socks and its too tight width wise. Longer doesn't = wider. I wear thin coolmax and a pair of heavy wool or alpacas. When temps go below 20 I add heat pacs. 0 or below I add 2 toe pacs,top and bottom. Most of my deer hunting is from a stand, 4-8 hours at a stretch. Small game and moving, no cold feet problems.