I’ve got a rule of thumb for picking via sizes, but I don’t remember ever being taught how to choose a size for my vias.
In general, I use vias with 0.5 mm diameter holes and 0.9 mm diameter pads. For traces larger than 0.75 mm, I size the hole to be about 75% of the trace width, with the pad diameter to be a little less than double the hole size. Of course, I’ll do my best to use as few different drill sizes as possible as well.
How about you? Have any guidelines for choosing your via sizes?