Recent developments of new therapy techniques using small photon beams, such as stereotactic radiotherapy, require new detectors to precisely determine the delivered dose. The dosimeter has to be as close as possible to tissue equivalence and exhibit a small detection volume compared to the size of the irradiation field, because of the lack of lateral electronic equilibrium in small beam. Characteristics of single crystal diamond (tissue equivalent material Z = 6, high density) make it an ideal candidate to fulfill most of small beam dosimetry requirements. A commercially Element Six electronic grade synthetic diamond was selected to develop a single crystal diamond dosimeter (SCDDo) with a small detection volume (0.4 mm$^3$). The Monte-Carlo code PENELOPE (parallelized version) was used to optimize both geometry and packaging materials of the diamond detector in order to respect the tissue-equivalent requirement. SCDDo dose profile and output factor (OF) measurements, were performed for small photon beams with a micro multileaf collimator m3 (BrainLab) attached to a Varian Clinac 2100 C linear accelerator. These measurements were compared to different detectors, both active and passive: silicon diode (PTW 60017), EBT2 radiochromic films, PinPoint ionization chamber (PTW 31014). SCDDo presents an excellent spatial resolution for dose profile measurements, due its small detection volume. OFs obtained with SCDDo are very satisfactory from 0.6 × 0.6 cm$^2$ to 10 × 10 cm$^2$ field sizes, compared to PinPoint ionization chamber which underestimates OF values in small beam.