We report on the structural and magnetic properties of nanoparticles of NiGa2O4 and 5 at.% M doped (M = Mn2+, Cu2+, Co2+, Fe3+ and Tb3+) at Ga site of NiGa2O4, synthesized by gel-combustion method. The particle size, as investigated by X-ray diffraction and transmission electron microscopy, could be fine tuned by a controlled annealing process. Weak ferromagnetism becomes significant, when the particles are in the nano regime (5-7 nm). The magnetization becomes insignificant at larger particle size ( 150 nm). Cu2+ and Tb3+ doped NiGa2O4 nanoparticles showed relatively large room temperature ferromagnetism compared to other doped (Fe, Mn and Co) and undoped NiGa2O4 samples. The weak ferromagnetism observed in the nanoparticles of NiGa2O4, which is antiferromagnetic in the bulk, is due to the surface disordered states with uncompensated spins.