Comment by jmkb

The limit is imposed by the OSM API itself, and it's a matter of how much data is already mapped in the area you're requesting -- a max of 50,000 nodes according the API docs. Of course data density varies wildly across the planet, so you can expect to fetch a lot more acreage in rural areas than in urban ones. But there's no way to know if a particular bounding box call will succeed without trying it out.

Editing software could be generally aware of high and low density locales and advise accordingly. And it could subdivide a large request into smaller ones. But the JOSM developers prefer to offer a closer-to-the-API experience.

[0] https://wiki.openstreetmap.org/wiki/API_v0.6#Retrieving_map_...