What size are the screws used for water bottle mounts etc?
Strictly speaking the answer should be M4, M5 or M6.  This specifies the threading as well* (M4x0.7, M5x0.8 and M6x1 is implied, where the thread is given in mm/turn).  The hex key for turning them will also have a size in mm, always smaller than the diameter, by how much depends on the head shape.

I would say M5 are the most common, based on a very small sample size.

*There are technically fine and extra fine metric threads, but you won't find them on a bike or in a hardware store.