With straight and wavy hair, you need to have 6 inches of hair before you start locking it. That said, if you have a small portion of short hair but the rest of your hair is long, go ahead and put that longer hair into locks. The short hair will have to stay out because it is dang near impossible to get shorter hair to stay locked at that point. For extensions, you’ll need good skill and a MINIMUM of 3 inches of hair, but that is very difficult to do. Not impossible, but difficult.
For coily and kinky hair, you can start locks with 2-3 inches of hair usually (I’ve seen shorter) and I think extensions can be wrapped in from the getgo by a professional. (I am not 100% certain on that, so take it with a pinch of salt).
I had an A-line bob when I started my dreads two years ago and the bottom simply would noooot stay locked at all. I waited a few months until it was long enough and got some extensions in and that was that. It was much easier with more length.
I hope this is helpful to you.