Paper-based microfluidic devices are a rapidly emerging field, but applications are limited by the relatively large size of the features that can be currently manufactured. Here, we investigate reducing the feature size by spray-coating cellulose nanofibers (CNF) onto patterned molds, with a focus on optimizing the cellulose chemistry and size to optimize molding performance. We investigated the effects of (i) decreasing nanofiber size and increasing aspect ratio by homogenization, (ii) decreasing the suspension viscosity by partial substitution of carboxymethylcellulose (CMC), and (iii) partial substitution of cellulose nanofibers with cellulose nanocrystals (CNC) to reduce aspect ratio and improve packing. Only CNC addition substantially improved molding fidelity. Channel widths down to 5 μm could be made using a 50:50 mixture of CNF:CNC. This is by far the smallest channel width that has been achieved with any cellulose fiber-based microfluidics and opens the way to further miniaturization within these devices.