Choosing the correct size for Vias is not always easy.
There isn't a straight forward answer to this as it depends on the type of board you are designing and the density of it, the current carried by the tracks, and so on.
The general rule is that you need at least 10 thou more than the drill size for the pad land diameter.
Also remember that Vias in the design should be specified as vias and you shouldn’t use free pads as Vias. This is essential otherwise the Design Rules Check (DRC) will not recognise them, and you may get the wrong results when producing solder mask plots.
If you are in any doubt, then check with your manufacturer before proceeding too far.