first you might try searching as i wrote something maybe a year ago...
my girlfriend is 5'1 1/2" and we have now bought 2 bikes for her in the last 2 years (the first was stolen).
the main problem is that most bikes are "designed" for a generic sized person - say a 5'9" guy... and then most companies don't start over from scratch for very small or very large bikes, but rather just shrink or expand a few key components and then twea few small adjustments for smaller/larger riders... but many aspects are then non-ideal.
#1 problem: for REALLY small frames you still have 26" wheels (usually) so this changes the geometry... (think bottom-bracket height)
#2 problem: the crank-arms and brake levers and shifters are usually not downsized...
(e.g. if you are 6'2 and ride 175mm cranks, at 5'1 she is 82% of your height, so proportional crank length is around 144mm --- but a "short" crank is 170 and a really short crank is "165")
a regular "small" generally doesn't work well as often only the frame is smaller and nothing else...
anyhow, a few companies do make "women-specific-bikes" and it's probably wise to check it out... the main problem here is usually (b/c of cost/demand) they usually only offer one or 2 models (e.g. specialized discontinued the Epic WSD and now has the Stumpjumper is 2 price levels -- compared to about 9 price levels for the non-WSD models)
what to look for:
1) (the obvious) low stand-over height to allow mounting/dismounting
2) low bottom-bracket or 24" wheels -- (to my knowledge Terry is the only maker designing to 24" wheels --- we didn't go further in researching here, but a MTB DESIGNED around 24" wheels would be the ideal -- a 26" for someone under 5'2" is ALWAYS a COMPROMISE!!). for a 26" wheel bike for a very short person you also need a significantly LOWER bottom bracket than someone your height -- e.g. the Santa Cruz Juliana we got has a 299mm BB (most XC bikes are in the 325 range)
3) adjustable brake levers
4) shorter crank arms -- we got 165mms which seem ok -- anything shorter is hard to find and $$$ -- but 170 or 175 are just too long!!
6) if MTB will be used for technical descents, then bike must allow for the seat to be significantly lowered to allow her to "get behind" the seat --- this means the top tube must not be too long!
some of the best models:
Santa Cruz Juliana (our latest choice - awesome although $$$$)
Specialized WSD - in Dec 03 we bought a 2004 Epic for her (she was happy with it before it was stolen) but Specialized discontinued the WSD Epic for 05 i think and now have a Stumpjumper FSR WSD which is ok
--- the Juliana has a lower BB than the Epic, allows here to lower the seat more and allows her to get behind the seat
Cannondale makes a WSD
i read recently that Kona will be making a "Womens" bike i think from their Kikapu XC bike
Thanks for any tips you can give me. I'm 6'2" so this is new terrority for me. hehe. Thanks for any help you all may have getting my life partner interested in sharing my bike passion.
the most important thing is that it fits and that she is comfortable riding it. e.g if she has normal brake levers it will be uncomfortable to always reach for the levers all the time or if the bottom bracket is too high she will not be able to put her foot down while still on the seat (assuming the seat is at the "proper" efficient-pedalling height) or if the top tube is too long she will sit uncomfortably AND not be able to get behind the seat for technical sections)
unfortunately there are very few "entry-level" WSD bikes... so you're stuck more in the $1000 to $3000 price range --- and almost no bike dealer has test/demo bikes in this size, so often you have to "do the numbers" and then order sight-unseen (we did this for the Epic and i measured and calculated and it worked out ok, but for the 2nd bike we WERE able to find a size Small Juliana demo bike -- Bicylce Sport Shop Austin Texas)