I'll use any size I have on hand, thaw, peel, and chop them into 1/4" long pieces, season with a little liquid crab boil, then fold them into the batter right before putting it in the hot cast iron.
This is the link I used initially, I'm not a sweet cornbread person and since this was Jiffy based it was a little sweet for me but it was still fantastic. I'm working on adjusting the ratios for my savory cornbread. It has regular onion, not green onion, but I added a little green anyway in addition to the regular. https://www.realcajunrecipes.com/recipe/shrimp-or-crawfish-cornbread/
Here's a pic from my last batch.