The scale size is determined by its outside diameter (OD) compared to real world braided line since an inside diameter isn't useful in scale modeling. See the tables in the section called LINKS, INFO & TECH.
Equivalent sizes at each scale:
1/25 scale = 1/2" (AN -8)
1/16 scale = 1/4" (AN -4)
Use Pro Tech Fittings sized at .025.