Porous silicon films displaying a distribution of pore dimensions can be generated by electrochemically etching silicon in aqueous ethanolic HF using an asymmetric electrode configuration. The median pore size and breadth of the size‐distribution in the film can be set by adjusting the HF concentration, current density, and position of the counter electrode relative to the silicon electrode. Films with pore sizes in the range of a few nanometers are used as size‐exclusion matrices to perform an on‐chip determination of macromolecule dimensions. The test molecule used in this study is bovine serum albumin (BSA). Optical reflectivity spectra of the thin porous Si films display distinctive shifts in the Fabry–Perot fringes in regions of the film where the pore dimensions are larger than a critical size, interpreted to be the characteristic dimensions of the protein. Gating of the protein in and out of the porous films is demonstrated by adjustment of the solution pH below and above the pI (isoelectric point) value, respectively.