Procreate never shows this in pixels: Brush Studio → Properties → Maximum size is a relative slider too. Measure your brush once below.
To measure: Actions → Canvas → Drawing Guide → Edit Drawing Guide → 2D Grid, set Grid Size to 10 px. Draw one straight stroke with firm pressure at a known size, zoom in and count squares across the widest part.
Type in any box — the others follow. Assumes the size slider scales width in a straight line from 0 to the maximum; a brush's Minimum size and pen pressure change real strokes, so confirm critical sizes with a test stroke.