Theory and design of a new electrical-mobility based instrument for measurement of aerosol particle size distributions in real time is presented. Miniature Electrical Aerosol Spectrometer has a rectangular cross-section with two main regions: the Electrostatic Precipitator (ESP) and Classifier sections. The ESP section enables charged particle injection into the classifier section in a narrow range of streamlines at the desired location. The injected charged particles are then segregated based on their electrical mobility in the classifier section and collected on a series of plates that are connected to electrometers. Real-time particle size distribution measurements can be inferred from the electrometer signal strengths with the knowledge of the instrument transfer function. A theoretical approach is developed to calculate Miniature Electrical Aerosol Spectrometer transfer function considering the non-uniformity in the electric and flow fields inside the instrument, and accounting for the instrument dimensions and its operating conditions. The theoretical predictions of size classification characteristics are seen to compare well with numerical results. The modeling results suggest that an optimal operational domain exists for Miniature Electrical Aerosol Spectrometer.