Starter Locs - Two Strand Twist Method

So I've been considering starting locs with two strand twists. My only problem is that i don't want too much shrinkage. On the shortest side of my head, my hair reaches my collarbone and I have a large head. I want to be able to get most, if not all of my hair in a ponytail with them in.

So my question is, is there a way I can keep my hair somewhat stretched to maintain length while locking up?

I really want to lock my hair but I also really wanna keep my length.

I have 4b and a little 4a hair. Possibly a little bit of 4c as well.

If this helps I have fine hair strands/high density (meaning a lot of hair strands)/low porosity.

****If you've seen this post before it's because I wasn't sure which category to post it in so I'm putting it in wherever I think it might fit.