Sizing
Thick size never reaches the core. It coats the surface, sheds at the drop wires, and leaves the yarn as unprotected as it started.
— µm
How far size reaches into the yarn in the immersion time
Washburn assumes steady laminar capillary flow into a wettable pore — real size boxes add immersion roller pressure, which helps, and air trapped in the yarn, which does not. Tortuosity is the fitted term carrying all of that, so calibrate it against cross-sections of your own sized warp before trusting the viscosity limit.
Size Liquor Viscosity & Yarn Penetration Depth Predictor — free, with the formula and a worked example, at Textile School.