Are there masks that will reduce polyurethane's VOCs?
Yes, their are things that help with polyurethane fumes, masks with volatile organic vapor cartridges, usually activated charcoal.

VOCs are not particles of "stuff" so a particulate filter (N95, P95, Etc) does not help with them.  This means a typical size in microns is also irrelevant because you are dealing with molecular size when it comes to vapors.

The particulate filters help with solids like dust and dirt, but not vapors.

Organic vapor cartridges absorb the vapors in the cartridge like a sponge absorbs water.  A sponge does not clog like a filter but gets saturated.  An organic cartridge also gets saturated by vapors and eventually needs replaced; but not because it is clogged.

You use a pre-filter with organic vapor cartridges to capture the particulates (solids) before the vapor cartridge.

I use a 3M half face mask with a 3M organic vapor cartriges and P95 prefilters for things with VOC's.  I can spray cabinets with laquer (lots of VOCs) inside and not smell a bit of the fumes where my wife walks in and almost gets knocked over by them.  Since paint has VOCs we also use the same organic cartridges when spraying the interior of a house.

The mask can also take particulate filters so we use those as well when in really dusty environments for prolonged periods of time such as during demo during a remodel.  Remodel dust does not have any dangerous vapors (they have already vaporized) so we do not need the bulkiness of the organic cartridges.