How to size a Mt. Bike for my wife?

My wife and I are looking for a replacement "mountain" bike for her to commute to work on...we're looking at offerings on craigslist.

we feel confident about being able to assess machines we look at, but I am not sure how to size a mountain bike for her..

such as, if an ad sez 21" frame or whatever, would that be the right size for her?
I know there's a relationship between a person's inseam and bike frame sizes, but I think mt. bikes size differently than road.

Obviously, she's going to ride the thing before she buys it, but before we set up appointments and drive around, we'd like to know whether a given bike (size) is even worth consideration.

thanks for any tips!

gd

I can't help you with sizing, but I can warn you about Craigslist. It's a wonderful resource, but be careful with what you buy, especially when it comes to bicycles. Selling stolen bikes on Craigslist is all too common. You'll see the ads, $2,000 road bike and the seller can't even tell you what size the frame is. I had one guy describe a carbon fiber frame as "graphite". Red flag.

There are a lot of legitimate sellers that use it, just be careful. You don't want to get a great deal on a bike and then have to give it back to the rightful owner. You also don't want to support the jerks that steal our bikes.

Just a friendly heads-up. Good luck!

__________________
"A gentle answer turns away wrath, but a harsh word stirs up anger." Prov. 15-1

thank you..no, we're all too familiar with the $2000 bike for $150 scams out there...

we're looking for the older used steel bikes which are price-appropriate for what they are..

same goes for computers and anythying else out there where it's a brand new $4K rig still in the shrink wrap for "only" $1300..

LOTS of scammers out there..


gd