If you want to be able to straighten your hair with a flat iron, how long should your layers be?

I have been growing out short layers for a year now. My shortest layer near the front of my face is almost chin length. Most of my hair is below my shoulders. I used to get my hair shaped around my face, and I would often straighten it. After some goofball hairdresser added shorter layers (I didn’t want them), I wasn’t able to straighten my hair.

I have been wearing my hair up for awhile because I haven’t had a proper cut since June of last year. I am wondering if the layers are long enough now, so that I could get it cut and wear it down?