I use bitmap fonts on windows at times as well, and I don't know of any workaround. Bring up terminal 8x12 in the windows font viewer, and it will only show point sizes for 12, 18, 14, 18, etc... Either because the bitmap font only has glyphs defined for those sizes, or possibly because it's only defined at 12pt, and the others are the 12pt size scaled up by a round factor so it doesn't look horrible. I'm not sure why Qt was reporting all of the other in-between font sizes.
Tim: I'm a fan of ProggySquare. But it's also bitmap font that's only defined at 12pt size, so the scaling will burn your eyes if you try to make it bigger.