Jerry's point about a "keep alive" circuit is spot on.
But most switch ratings are for when the switch is opened under load so the contacts don't burn from arcing. Unless your starter is engaged while you're turning the battery switch off, you shouldn't have to worry a lot about switch rating. Although a switch that has terminals with a #10 screw size would probably be toasted the first time you hit the starter, engaged or not. If you use a disconnect with terminal screws at least as big as the lug on the starter, you should be fine.
