It's easy to remove the soft felt from the base and put both in the dishwasher for easy cleaning! Alternatively you can rinse them with warm water and dry.

Yes the whole base including the bottom is silicone. The bottom is a slightly different texture to make it non-slip. 

Yes absolutely! Although you probably wouldn't want to leave them outside continually in all weather. 

3.54 inches internal diameter and 4.13 inches overall.