Here's a great online pinhole camera calculator to help you figure out optimal pinhole size"
http://www.mrpinhole.com/calcpinh.php

here are needle numbers converted to mm:
4, .90mm
5, .80mm
6, .73mm
7, .66mm
8, .58mm
9, .51mm
10, .46mm
11, .40mm
12, .35mm
13, .33mm
14, .30mm
16, .25mm

if you're looking for more info, may i suggest the f295 pinhole forum:
http://f295.tompersinger.com


cheers!
Tom