The sand or dirt you have in it should be 12" deep, the tub itself can be a little taller or a lot taller, so long as she has 12" of something to dig in.
I use a kitchen trash can, but I think the recommended minimum is something like 12"t x 7" x 7", but a little bigger is probably fine as well. I use a big one because my veiled won't go into the bin on her own, so I have to abandon her in it when I know she's ready. She'll fuss for a bit and then lay eggs, so she's stubborn.
Sand is recommended, but I've used moist Ecoearth with success, so I prefer that personally (it's much lighter to carry around if I need to move the bin, that's for sure) but you can also use dirt so long as it is fertilizer free and free of the little white balls. Just plain dirt.