A microencapsulation process for preparing protein-loaded microspheres based on a solvent-extraction method was scaled up using continuously stirred tank reactors (CSTR) from 1 L to 100 L in batch size. This study was concerned with developing a quantitative correlation between the size of the microspheres and process parameters. The process parameters considered include operational variables and physical properties associated with both the dispersion and dispersed phases. Dimensional analysis was used to establish such a correlation based on protein-free poly(lactic acid-co-glycolic acid) microspheres in an oil-in-water emulsion system prepared in a 1L CSTR. This correlation was found to accurately describe the preparation of protein-loaded microspheres in a solid-in-oil-in-water system. Poly(methyl methacrylate) was found to behave similarly to poly(lactic-acid-co-glycolic acid) and could be used as a model polymer for scale-up investigation. This study showed that dimensional analysis can be used to predictably scale the current microencapsulation process up to 100 L to produce particles of defined size.