Figure out how fast your recordings eat up drive space, and how big a drive you actually need. Runs entirely in your browser — nothing is sent anywhere.
Recording locally and streaming to a platform use completely different bitrate budgets, which is the main reason people are caught off guard by how fast a drive fills up. When you stream, your bitrate is capped by both your upload speed and the platform's limits — Twitch tops out at 6,000 kbps no matter what. When you record locally, none of that applies: there's no upload involved, so OBS defaults to a much higher bitrate for better quality, often 2-4x what you'd ever stream at. That's great for editing later, but it means an hour of 1080p60 footage at "best quality" can land anywhere from 3-6GB, while the same hour at streaming-grade bitrate would be under 3GB.
The four things that actually move the needle on file size are resolution, frame rate, bitrate (which the quality setting above controls), and how demanding the audio is. Going from 1080p to 4K roughly quadruples the raw pixel data, and 60fps versus 30fps roughly doubles it — so 4K60 recording can eat drive space five to ten times faster than 720p30. Audio is a much smaller factor by comparison, usually only a few percent of total file size either way.
A few ways to cut storage use without touching quality much: record at "smaller files" instead of "best quality" if you're not planning to do heavy editing (the difference is often hard to spot on the final upload since YouTube/Twitch re-encode it anyway), record only your local capture and skip a separate "backup" recording if OBS is set to do both, and clear out VODs you've already exported or uploaded elsewhere rather than keeping local copies indefinitely.