As other's have stated, fonts are bitmaps to begin with. They are vectors. So there is no truly native size.
However, most digital fonts can come with something that is called 'manual hinting'. This means someone went in and made on-screen bitmap copies of each glyph by hand.
So there may be one or more font sizes with this font than will have 'native hinting' for that size.
The catch is that the software has to obey it and there's less of a guarantee today that that will happen. Many operating systems and web browsers can apply their own font smoothing and this is often better (or at least as good) as the old-school hinting files. Some have to apply their own smoothing as they are on high density screens. Of course, as you're probably finding out, a bit-map looking font is something you don't want font smoothing applied to.
If this is a bit-map looking file, then you're probably just best setting it in a few common sizes and see which one looks best. The one that looks best is likely the size(s) that had manual hinting files created. Try 8, 10, 11, 12, 16, 18, 24 pt sizes.