If I recall correctly, the astronauts are in more danger from secondary radiation sources (the hull or shielding becoming radioactive) than they are from primary solar radiation. Biological materials are transparent to most solar radiation, but metal is not.
A solar flare would be a different proposition, however.
So a relatively small radiation shelter would be sufficient, provided the astronauts have sufficient warning. Since the dangerous particles travel at less than the speed of light, there should be ample warning while they are in flight.
Folks roaming around on the surface could require more time to get to shelter.
Edit: Coronal mass ejections are another danger, but the types of radiation appear to be similar to solar flares.