Two megabytes covers the YouTube thumbnail limit and the default upload cap on many CMS installs. Most camera JPEGs exceed it because they are saved at quality 95 or above; re-encoding at 85 usually halves the size with no visible difference. Photos stay at their original pixel size unless they are truly enormous.
At 2 MB you can expect full camera resolution, 3000 px and beyond. The exact number depends on how much fine detail the image has: skies and plain walls compress well, foliage and fabric don't.
Tip: For YouTube specifically, use the YouTube thumbnail page: it also crops to 16:9 at 1280×720 or 1920×1080.