I would rather wear my own skirt than use one from the monastery, but I'm wondering how long it has to be. Can it be below the knee or mid-calf, or does it have to go down to the ankles?

The Sacred Destinations website says women must wear long skirts. Other sights seem to indicated that longer than knee length may be enough. Do you want to risk wearing one of their skirts if yours is too short?

I took a mid-calf, black "broomstick" skirt (one of the crinkly, full ones that you can twist) to wear. It didn't weigh much or take up much room and looked appropriate for Greece with a colorful top. Any women wearing pants, shorts, capris, etc. were given pareo/sarong-type skirts to wear over their clothing. I think that knee-length may work too, but why chance it?

I wore a mid to lower calf skirt.

Mid calf will be fine. I was wearing capris which they weren't happy with and I was given a skirt to pull on over the top. It was only an inch or two longer than my pants but as long as they were happy, I was happy.

It's a fantastic place, I hope you love it.

Also remember your shoulders will have to be covered, so consider what sort of top you will wear. I wonder if it is worth the trouble to pack a long skirt just for Meteora. If you plan to wear it elsewhere, fine. Perhaps a sarong, in conservative colours, will suffice, and it will be useful throughout the trip.