Truetype fonts are scalable, and your font does not look like a fixed width font.
You can arrive at a suitable value for the multiplier on the glyph tab of the image import dialogue by trial and error. Its not critical, as long as all glyphs are scaled by the same factor. The glyphs can be scaled using a transformation later anyway. What is more important, is the resolution of the bitmap to get sufficient detail.
You might want to try the method in my tutorial to speed things up once you have decided on the right multiplier. Aim for a Caps Height of about 1500 to 1600 for your font, which has shorter descenders than the default New Font template font.
When you import a bitmap, it is positioned at 0,0 on the grid. You will need to adjust side bearings to give your font some spacing. You can use Autometrics from the Tools menu to make them all the same initially, but the shape of the letters affects the white space that they need on each side, so some manual adjustment is always necesssary. Lowercase i and l, for example, need more white space than capital O, V or W.