I didn't understand Chaco's M8 designation. Looking at the companies website, I see now that "M" does refer to men's  but I'm a size 9 street shoe and I went a little larger with these Chaco's. The #8 in raised letters on the heel though is baffling. So I've given the foot bed measurement.

Up Date - From the chat line with Chaco: the M means it's a mens, the 8 means it's a size 8. Our sandals run a bit big, so it's no surprise you had to go up that far. 

Foot bed measures 101/2", big toe to heel. Overall in very good condtion. There is some webbing wear on the toe strap behind the big toe on each sandal. Repairable with Seam Grip and webbing. Soles have much remaining life. Asking $25.00 plus shipping cost from 80517. Please pay by PPFF or add the goods fee if that's your preference.

Right Foot - Left is not as frayed.

Right foot - Left is not as frayed