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One thing I've never seen is a chart comparing the different pen sizes. I don't know just how helpful this is, since tiny differences in pen girth = huge differences in the feel of the pen.

1-Pocket Pen 2-Slimline (modified) 3-Sierra 4-Pentel Conversion 5-European 6-Tall Slimline 7-Jr Gentleman's 8-Jr Majestic 9-El Grande This isn't comprehensive by any means, though it can be used as a basic guide. If I say it's "Number 9 sized" then you know it's the thickest of the lot.

I hope this helps!