Answer:
Beta particles are smaller.
Step-by-step explanation:
When an unstable nuclei will break into stable nuclei then the nucleus energy of the unstable nuclei will be given to the the fast moving beta particles.
These beta particles are same as electron as they are negatively charged whose magnitude of charge is same as that of electrons charge.
so we have

now since electrons are smaller in size as we compare it with alpha particles so we can say that the beta particles can penetrate to more depth then the alpha particles
so here correct answer would be
Beta particles are smaller.