In my experience Salvias don't have really deep roots, altho' that may not be true of some species. I've had it self-seed - and survive - in a shallow pot, maybe only 2-3" of soil.

The roots of plants grown in containers will grow to fit the container, but when grown from seed in the ground the roots will go wide.

I would think a 5 gallon would be more than sufficient for any but the hugest of Salvias.

Thanks.