Then editor of The Magazine of Fantasy and Science Fiction, Kathryn Rausch, wrote in an editorial that she would only consider submissions printed using using 9 point type or larger. Others may prefer larger type, so consider her standard as a guideline.
Along with type size it helps to consider the font in question. Some fonts do not read well at small sizes, others were designed to be read at small sizes, and so don't read well at larger. Monacoand Geneva start to blur and smear below 9 or 10 points. Chicagoand Times, being designed for printing at very small sizes (4 or 5 points), look horrid at 12 points or greater. So if you really must use 8 point type, I'd recommend Chicagoor Times over any other available font.
Hope this helps