I dont always do a watercolor sketch before doing a painting. But generally I do it if I am not very conversant with subject or I have not been able to resolve the subject in mind. And sometimes I do it just for fun.
I never use fresh paper for these sketches. I use the unpainted side of paintings which have not made the cut (which gives me quiet a bit of paper to play with. ;)). And I do them quick and sometimes in a couple in parallel. And these are never bigger than half of a quarter sheet.
Here are few WC sketches. I'll post a few more in my next post.