Introduction: Variable resolution x-ray (VRX) CT is a new type of CT that can image objects at various spatial resolutions. In a VRX CT scanner, the spatial resolution increases at the cost of reduction in the field of view (FOV). An important factor that limits the spatial resolution of the VRX CT is the effect of focal spot size. Also, the optimum magnification is different at each incident angle. Material and Methods: To investigate the spatial resolution and the FOV in VRX CT, we used the numerical parameters of an actual VRX CT scanner. The effects of decreasing the focal spot size and optimizing magnification on the spatial resolution and the FOV of the system were studied theoretically. Results: The focal spot size was found to greatly limit the spatial resolution of the VRX CT at the small incident angles. By decreasing the focal spot size, the spatial resolution was comparatively improved. Optimization of the system magnification without decreasing the focal spot size could greatly increase the spatial resolution of the system. After optimization of magnification, the FOV of the system decreased significantly. Discussion and Conclusion: The spatial resolution of the system after optimizing the magnification increased from 7.5 to more than 35 cycles/mm and also the effect of the focal spot no longer limited the spatial resolution at incident angles smaller than 13˚. The disadvantage of optimization of magnification was a great decrease in the FOV of the system. The optimization of magnification has the reverse effect on the spatial resolution and the FOV of the system so at each incident angle, according to the importance of spatial resolution or FOV, the system magnification can be set to lead to the desirable condition.