For real-time computed tomography (CT), a temporal resolution lower than 30 ms is required, which cannot be delivered by conventional CT architectures with a rotation gantry. Stationary CT architecture can achieve a temporal resolution lower than 30 ms by eliminating the physical rotation of the gantry and by electronically sweeping X-ray beams across the gantry. Stationary CT architecture utilizes two separate arrays, one for distributed X-ray sources and another for detectors. These individual X-ray sources are required to be compact, fast, and individually addressable in order to acquire 200+ projections for successful CT reconstruction and to achieve a temporal resolution lower than 30 ms. A compact X-ray tube that fits these requirements is being developed. The aim of this paper was to design an electrostatic electron focusing lens for the first-generation prototype compact X-ray tube primarily consisting of a carbon nanotube-based electron source and a transmission-type anode by studying the multiple variables that affect the focal spot size (FSS) and by comparing different types of electrostatic lenses with the help of simulations. This multivariable simulation study was conducted using an object-oriented particle-in-cell code, OOPIC PRO. The results of these simulations showed the following: Einzel lens can achieve the required FSS (lower than 1 mm); lens aperture, thickness, and location of the lens can be used as a coarse control over the FSS, whereas the lens potential can be used as a fine control; and the chromatic and spherical aberrations can be reduced if Einzel lens is used.