I've been playing around with large print sizes this week using 4x5 transparencies scanned at 2,400 dpi (my computer couldn't command enough memory for higher resolutions). I found that the printed scans stood close but not microscopic examination with 60" wide prints which equates to 200dpi which is 12x although I might have been limited by resolution of the scan. The recommended 300dpi would bring me down to a 40" print which is the same as a 12" print from 35mm which is as far as I was ever happy with from a miniature camera. The biggest I've had traditionally printed have been 20" (they're so expensive).

Woolliscroft really should have a go with 4x5. With five times the area of 6x6 there is a real difference and the slow pace makes you look carefully at what you're doing.