We present a high-finesse optical cavity containing a LiTaO3 electro-optic crystal, devoted to free-space electric field characterization. Theoretical considerations will show that the modulation depth is directly related to the transversal components of the field to be measured, thus opening the way to vectorial mapping of the electric field using a single electro-optic crystal. Also, a discussion about noise and sensitivity will be given. As the latter increases with the effective cavity length, and bandwidth decreases, a trade-off is realized, allowing us to measure an electric field of 60 mV/m/Hz^1/2 in a 110 MHz bandwidth. Cavity dimensions are less than 8 mm^3, giving an inner-crystal transverse spatial resolution of 70 µm and allowing pigtailed systems to integrate.