SIDI strives to accomodate riders with feet of every shape and size. From a Women's size 35 to a Mega 50 and everything in between, there's a probably a shoe or boot that will work for you.
Most SIDI footwear is built on molds or "lasts" with a men's D width, which is fairly standard. However SIDI does offer shoes and boots with a larger range of widths.
The "Mega" width is a Men's EE width on sizes 40-47 and a EEE width on all sizes above. This gives 4mm of extra width across the ball of the foot when compared to the standard D width. The Mega also provides for a higher instep and a wider heel cup, creating more volume throughout the the shoe.
The Narrow width is a Men's B width. This is 9.5mm more narrow across the ball of the foot and has an overall more snug fit with less volume.
The Women's lasts are made to mimic the natural taper of most women's feet. It is narrower across the ball of the foot with a B width. Overall there is less volume through the instep area and heel cup.
To figure out what last is right for you is to visit a SIDI dealer to be professionally sized using a Brannock device.