How do you know what the right stirrup length is?

I don't know if there is an article about this already, and if there is please forgive me! (You don't have to read below if you don't wannaa.)
Long story short.(hopefully)
My dad gave Jack away to the barn owner, and im good friends with her. Sooo, I texted her and asked her if I could start riding jack again.
She didnt reply so my dad called her.
She said that a LITTLE girl was feeding him at the moment and she's going to call the girls dad and see if I can start riding him again:)
But what made me mad was, little girl?
Seriously, if she's riding him she does not need to be.
Jack was more than enough for me to handle, and im 14.
So if im 14 what is little? 10?
Anyway, opinions on this? Jack was aggressive towards me at times and a little girl to be handling him? That really scares me.
Sorry for typing so much!
Please answer the topic question and opinions on my situation! :)
Thank you! ♥

How has that got anything to do with your stirrup length?

^ Dido?

Sorry XD
I feel like i've been using the wrong stirrup length when I was riding jack,
So if I start riding him again, I need to know the right length

Ooh okay. Do you ride Western or English?

English ;D
Hence, why I posted it in the english section;]
Ommg that was rude! D: im sorry xD

Haha that's okay . I usually just hang out in the "new posts" section, so I don't know what section I'm posting in lol. To measure mine for flat I put the iron to my armpit, and when I make a fist the buckle should be at my knuckles. For jumping I think you're supposed to bump thestirrups up about two holes.

Thankyou

My trainer has told me it needs to hit your ankle.

Thankyou