As stated above, get .22 micron. That is the USP standard for sterility. Also try to get the largest diameter, e.g. 25mm. More surface area=lessclogging.
Some people prefilter with .45 to extend life of .22.
As far as sterility, just push some BA through the filter first. When it likely your are using questionably sterile/clean glassware and doing all this in the open air, sterile filters would be overkill and not cost effective.
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