Required dress code at monasteries for women

In addition to seeking advice here, I suggest Googling "dress code" with specific monastery names in hopes of finding recent reports from visitors to the places you intend to visit. I think there's a possibility there will be loaner garments available, but I can't guarantee that.

Generally covered knees and covered shoulders - both male and female. Sometime not enforced and other times strictly enforced.

Lonely Planet says that loaner wrap-around skirts are usually available at the entrance and I now remember using them about five years ago. Find them in just a simple basket or rack.

As a general rule from observations at a number of sites, but not specifically in Greece.
1) covered shoulders
2) legs covered to the knee
3) carry a light scarf in case hair needs to be covered

As of about 25 years ago, some rural Greek monasteries had the strictest dress codes I've ever encountered in Europe. Full-length slacks did not cut it. Had to use a loaner skirt (if available). This may have changed in the intervening years, but I would not depend on information that is not site-specific.

When I was there it was a long loose fitting skirt ankle length shoulders covered for sure but not sure about arm length. I always have a wrap so not sure. I do remember they had skirts you could use and some places but not others. I would add a long skirt. I agree it was the strictest dress code in Europe

It is definitely worth it to throw in the skirts. You will sometimes not be permitted with shorter skirts/shorts if knees aren't covered especially. The idea is coverage, though, so there has never been a problem when I have worn jeans or pants--just make sure they aren't leggings or super tight. You will sometimes visit monasteries that are more relaxed and sometimes see other tourists breaking this rule, but frankly it is often considered really disrespectful to do so even if it isn't explicitly called out. So, in short, bring clothing that will cover knees and most of the leg, either skirt or pants.

I visited the monasteries of Meteora a year and a half ago and there were baskets of loaner skirts at the entrances. And, just wearing pants that cover your knees is not enough. The sign outside Grand Meteora Monestery states, "Shorts are not allowed. Ladies in sleeveless dresses, slacks or pantaloons will not be admitted."