The lithium niobate crystal optical electric field sensor has the advantages of wide bandwidth and negligible interference to the original electric field due to its all-dielectric structure. But it has low sensitivity to measure the electric field. The influence mechanism of crystal geometry size on sensor sensitivity is analyzed. The sensitivity of the sensor is improved by increasing the crystal size along the direction of the applied electric field and reducing the crystal size along the vertical direction of the applied electric field on the crystal cross-section. The influence of different thickness, width and length of the crystal on the internal electric field intensity has been analyzed using COMSOL simulation, and it is concluded that the internal electric field intensity of the crystal increases by about 5.1 times and 12.3 times when the thickness decreases from 15 mm to 3 mm and the width of the crystal increases from 3 mm to 22 mm, respectively. When the length of the crystal increases from 15 mm to 55 mm, the internal electric field intensity of the crystal changes by only about 5%. Three lithium niobate crystal electric field sensors with crystal sizes of 3 mm×3 mm×42.2 mm (x×y×z), 3 mm×6 mm×42.2 mm (x×y×z) and 6 mm×6 mm×42.2 mm (x×y×z) have been designed and developed, the sensitivities are 0.243 mV/(kV·m^-1), 0.758 mV/(kV·m^-1), 0.150 mV/(kV·m^-1), respectively. When thickness and length of the crystal are constant, the sensor sensitivity is increased 3 times with the crystal width increased from 3 mm to 6 mm. When width and length of the crystal are constant, the sensor sensitivity is increased 5 times with the crystal thickness decreased from 6 mm to 3 mm. Combining the simulation and experimental results, it is concluded that a higher sensitivity electric field sensor can be developed by designing the crystal with wider width and thinner thickness under a certain crystal length.