The default is 10Mb, but you can change it in Admin -> Attachments. Don't go too large though - Confluence isn't designed to be an upload/download manager or a file repository, and you can get timeout problems with very large files.
I've not seen anything significant up to 100Mb, so that's the highest I tend to set it before I start resisting the requests for higher amounts with any gusto. 250Mb is the point at which the problems tend to start in my expericnce - it's where I start telling people to use proper file storage and not to rely on being able to attach or read files with Confluence.
Oops, I also wanted to say that it's on a file-by-file basis - doesn't matter if you've got hundreds of thousands of large files, it's just that for any one selected large file, you can run into upload/download problems. Most of the time, even much bigger files will work. You just can't *rely* on the upload or download working.
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