Jamie,
Here's a photo Galanw posted a little while ago that shows the relative size of some of the more popular pens:
http://www.penturners.org/forum/topic.asp?TOPIC_ID=18084&SearchTerms=pen,sizes
I think a few others have posted similar photos, all very helpful.
As for the size - I write with a Baron or Jr Gent all the time and find them very comfortable. But a "fat" slimline, esp in acrylic, offers a lot more possibilities for fun in the design -- colors, beads and coves, custom centerbands, all of it.
Just a thought... you could give her a "Pen Of The Month" gift certificate and make a new pen for her every month! Get her involved in the design and color choices.... and you may soon have competition for your lathe!
cheers Eileen [8D]