The Homeworld series, a space RTS game, that has pretty decent scale, but I imagine it is hard to get your hands on a copy of it these days. You can also look at Sacrifice, a weird kind of mix of games that puts you on the ground as a magically powered commander and you can command armies of monsters in an RTS fashion.
Maybe the first Sword of the Stars game... though that is a 4X game, but the combat rounds have some RTS elements to them, but it is not super in depth.
