48px
A face earns its keep at the size you actually set it.
36px
Set the same sentence twice, once large and once small. The small setting is the one that tells you whether a typeface will hold up.
30px
Counters close, joints thicken, and the gaps between letters start doing more work than the letters. What looked confident in a headline can turn muddy three sizes down.
24px
Most of the type anyone meets in a day sits between 16 and 24 pixels: the body of an article, a form label, the terms nobody reads. A face that survives down there is worth more than one that only photographs well at 200.
20px
Read a few lines rather than a few words. Rhythm only shows up over a paragraph, and so do the things that get tiring: a narrow e, an ambiguous l and 1, an italic that leans harder than the roman, numerals that sit too high against lowercase. None of it is visible in a single word set large.
16px
This is the size where a typeface either works or quietly does not. Stems thin out, terminals blunt, and the spacing decides how fast anyone gets through a sentence. Check the punctuation while you are here, because commas and quotes carry more of the reading than their size suggests, and check a number or two: 0 against O, 1 against l, 3 against 8. If the paragraph still reads easily at this size, the rest of the family will usually take care of itself.
14px
Below sixteen the decisions stop being yours. Hinting, the screen and the reader’s own settings take over, strokes land between pixels, and a face that looked crisp one size up can go soft. Set a caption here, or a table, or the legal line nobody reads, and see whether the words still separate cleanly.