A 0.9mm hole doesn't give much tolerance but for a square pin, the corners can usually cut in a tiny bit without problems - the flats provide enough gap for solder flow. The actual diagonal of your 0.025" square pin, assuming no tolerance issues, is 0.035355" (0.898mm). If you say "0.9mm hole" then you will get a 0.9mm hole, measured inside the plating, so that a 0.9mm pin will be a snug fit. That is to say, any allowance for plating thickness, drill tolerances, etc. When you send your design data to a PCB manufacturer ("board house") they will nearly always expect that the hole sizes you give them are "PTH finished hole". You don't say whether you have much experience with electronics design nor how you plan to turn your design into an actual PCB, so I'm going to assume this is your first PCB that you're having made professionally. That figure is the hole size needed to accomodate the pins you specified.
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