Draw once in Inkscape, then get print-at-home tiles that are measured, layered and ready to sell, without rebuilding the pattern for each format.
By hand, tiling in Inkscape means a page per tile (Inkscape 1.2 and later has multiple pages) for each paper size, and the PDF loses the size layers.
Use Layers and Objects to make one top-level layer per size (S, M, L) and shared layers starting with an underscore (_grainlines, _notches, _test). Sublayers are fine.
Save as Inkscape SVG. Inkscape layers are exactly what Sewpress reads. Inkscape's own PDF export doesn't keep layers as PDF layers, so upload the SVG.
Since 0.92, Inkscape uses 96 units per inch, the same as Sewpress. Files made in older versions used 90, which reads as 93.75%; Sewpress spots that on the inspection screen.
Sewpress places your master once and builds tiled A4 and US Letter, measuring the test square in each.