Generally, I buy Women's 9 shoes.  I bought Men's 8 and planned to wear thick socks and skinny jeans and tuck them into the boots.  Worked perfectly.  The boots were perfect, too.  Mud was over 6" deep, ruts over a foot and my feet stayed clean and dry.  Boots were sturdy, stayed on, were comfortable.

I live in Phoenix, the mud was on a job site in VA...I left them behind for a colleague to wear since theirs weren't nearly as waterproof.  I'll buy another pair.  They're that great!