I'm going to buy a bicycle for my girlfriend. She's a shorty - only 4' 10" (I think she's 4'9", but she says 4'10" - okay). I'm looking at getting her a mountain bike - nothing extravagent or elaborate, just something nice for under $100 for now (as I don't even know how often she'll use it as she said she hasn't ridden one since she was around 13 - she's 33 now, but said she would if I got her one). This is going to be a surprise gift (early Christmas present), but I don't want it to be a surprise that it's either a) too small or b) too big.

I found a few on the internet that caught my eye (some Walmart, Sears, and Target bikes - I know.. but I'm on a budget right now, and besides, I don't want to shell out $200+ for a bike that may not be used), but I'm not sure what size should I get, 20" or 24"? Kids or womens? Anyone have any ideas?

I'm currently using public transportation, so stopping by a department store at this time (after work), is really hard for me.

Thanks for any help.