Probably M5 - but not necessarily. I replaced all the original screws on my bike with stainless cap screws (aljen bolts). They don't rust and allen screws are (usually) easier to undo than normal bolts in the smaller sizes. Particularly worthwhile was replacing the derailleur adjusters - as the original Phillips headed ones were made of cheese - these weren't M5 of course.
* M5 means 5mm diameter, and standard metric course pitch and thread form.
