this is one place where the more expensive ammo pays for itself... as stated before #4 buckshot is as large as you can use... as you shop around also look for BB, BBB, T, and F shot in copper plated lead... all those will work good on fox, and will anchor a coyote if they're close enough...
Federal Premium is my choice for ammo when I use the shotgun... it's hard enough to find 10ga but you can always get someone to order for you if necessary...