If she's so tight that you can't penetrate then she's probably got
vaginismus - a spasming of the muscles sometimes encountered if the woman is extremely nervous or anxious. It can be as impossible to penetrate a closed fist.
First of all you need to work especially hard to get a woman with vaginismus to relax. But that alone is rarely enough to work.
Then there are "dilators." You'll have to search for them online because they're medical devices (though I'm sure there's an equivalent adult toy). Dilators are simply a series of larger and larger dildos. You start by inserting the smallest during foreplay and then, as she gets more relaxed (presuming you know your foreplay), to exchange it for a larger and larger one until she feels she can accommodate your penis.
Her height has absolutely nothing to do with her capacity. The shortest girl I've dated was 4'11" and she had no problem with me (I'm about 8" and thick.) - but another 5'4" girl didn't even want to have sex at first because she thought it was too big (In retrospect her "get-that-thing-away-from-me" reaction is funny. But at the time it was awful). It's true that not all women lust for large cocks - in fact many are intimidated by it. It really sucks that you have to find this out only at the moment when you are in bed.
Another ex-girlfriend was so nervous she got .. vaginismus. It was frustrating as heck so I know how you feel.
BTW - don't make the mistake of blaming her for your frustration. Sometimes women develop vaginismus as a result of some sexual trauma or abuse in the past. So be kind.
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addendum: Here's a link for more info:
http://www.vaginismus.com/?gclid=CP7...FQzDYgodFSaAcQ