Question

Explain why the size of magnetic field determines the kinetic energy of the proton in cyclotrons?

Answer 1

Because without the magnetic field being strong enough the charged particles (protons in this case) will collide with the accelerator walls and be lost.

Explanation:

It is only the magnetic field that causes the particles to bend. As they go faster they require a stronger magnetic field to keep them at the radius of curvature of the accelerator. Their velocity is thus limited by the strength of the magnetic field.

Hope that helps, but the CERN website may be your next port of call.