I would write 'one half page' if you're saying it covers half of one page. You wouldn't use a decimal or a fraction.

Typically we would pronounce it as "zero point five pages." Hence, 0.5 pages is correct. However, "half of a page" is also correct. In general, unless the number is equal to one exactly, use plural. This still applies to my second example, as it is sort of like this: (1/2) * (1 page).