Beaumont Newhall's note at the end of Book I of the _Daybooks_ ("Mexico", p. 206), states that Weston made enlarged negatives from the 3x4 Graflex negs using a glass interpositive and that he "never printed by projection."
I don't think I've seen an original Edward Weston print larger than 8x10", though I've seen some that were obviously contacts from the 3x4 negs (sometimes cropped), 8x10" contacts, or 8x10" contacts from enlarged negs.
I can't seem to pin down that shot of Weston stalking through a field with his 3x4 Graflex SLR, but it's pretty well known and shouldn't be hard to turn up. There are some shots on the web with Weston and an 8x10" camera (Seneca, I believe), mislabeled "Edward Weston and his Graflex."