Here's an example of one that I did a while ago. (it's the only one I could find where I still had the original sketch)
It's a bit more on the "cartoony" side. But you can get the idea of the process anyway.
I do the sketch on whatever size it ends up being. (sometimes I end up taping letterhead sheets together if it runs over) Then I take the scan into Illustrator and trace the sketch. You can then take the vector lineart and scale it to ANY size you prefer! I then open that in Photoshop, place it on a "master line" layer that is locked. Then do my color layers below that.
(hope that makes sense!)
Not saying that this is the best way, just the way that works the best for me at the moment.
But, I am always open to hearing other techniques!