Sizable means noticeably large or considerable in amount, but not necessarily enormous. A sizable crowd at a school assembly might be 200 people, which is large enough to fill the gym and create real energy. A sizable donation to the library might be $500, not millions, but enough to buy dozens of new books.

The word sits in that useful middle ground between “big” and “huge.” A sizable improvement in your math grade means you've made real progress worth celebrating, even if you haven't jumped from failing to perfect. A sizable obstacle is one you'll need to take seriously and plan for, though it won't necessarily stop you.

You'll often see sizable in contexts where someone wants to acknowledge that something is genuinely large without exaggerating. A reporter might describe a sizable portion of voters, or a scientist might note a sizable increase in temperature. The word carries a sense of careful honesty: it's big enough to matter and big enough to notice, but the speaker isn't trying to make it sound more dramatic than it really is.