Ideal ferro rod length and girth?

Is bigger better?

 

Well you just want it to be wide and long enough that you can get a good firm grip while scraping down the .. um.. shaft.

 

Bigger is always better, until she starts complaining.

Given how few of these are ever needed, get one that fits wherever it is you plan to carry it. Example, if you're going to add it to your knife sheath, get one that fits your sheath. If you're just going to stick it in your pack, well get whatever is comfortable. The one thing I will say is that I have not had good luck with the plastic ends, they always seem to crack and break. With that in mind, I quit buying those and just buy the rods and eopxy on a piece of hardwood dowel or antler with a thong through it.

I can't remember the site right now and I can't find it in my bookmarks, but there is a guy that sells a huge selection of ferro rods that most of the "bushcrafting" crowd recommends and I'd highly recommend him as well. I'll have to look through my bookmarks again after dinner and see if I can find him but he sells them in sizes from tiny to something you could probably whack somebody with.

FWIW- I prefer a mini Bic lighter in the most obnoxious color, usually orange, that I can find.