Where the sizes come from
Printed dialogue reads comfortably at a cap height of about 2 mm on a standard 6.625 in page. Comic fonts have cap heights around 70% of the point size, so that is roughly 8 pt, with leading about 1.18 times the size. Bigger pages do not want proportionally bigger type, so the size scales with the shorter side of the trim to the power 0.6: a magazine page gets a little larger, a manga page a little smaller, and turning a page landscape changes nothing. Results are rounded to half points. If you letter at a larger working size, scale the numbers by the same factor.
Questions
What font size should comic lettering be?
About 8 pt at print size on a standard comic page, with leading around 9.5 pt. Manga-size pages use a touch smaller, magazine-size a touch larger.