Beam optimization of laser-accelerated protons is required to progress towards the development of applications, e.g. for fast ignition of fusion targets or dense plasma radiography. For this, three areas of improvement need to be pursued: increasing the maximum proton energy, enhancing the laser-to-protons conversion efficiency and reducing the beam divergence. Here we report on the reasoning which led us to envision limited size target to improve protons acceleration. We point out that pre plasma leakage from the target front side is a critical issue to allow efficient acceleration from such limited size targets and show that with very high temporal contrast laser pulses proton acceleration is improved in such targets.