Ordinary publications use fonts around 11Q to 16Q big for main text. The unit Q (級) is the standard measure in Japanese typesetting industry, being 1/4 length of a millimeter (Q is from Quarter). Thus 11Q = 2.75mm ~ 7.8pt, 16Q = 4mm ~ 11.4pt. The default font size of MS Word is set to 10.5pt in Japan.
Actually, since Japanese characters don't have ascenders and descenders like European letters, each letter is designed to cover up almost full body height, so that they usually look one or two sizes larger than European ones with the same nominal size. In exchange, we tend to need taller line spacing than European books. Optimal line height is 166% (12⁄3) to 200% of the font size. Compare page layouts of a Japanese and an American paperback.
The websites usually prefer slightly larger fonts than Western average if possible, because of the pixel density matter. For example, current text font-size and line-height of Japanese WP are set to 15.1px/24.2px while the English version is 14px/22.4px.