It can handle about 2 billion rows and columns. Aside from that, it's limited by your machine's memory. If you open a file in edit mode, the memory footprint is about 3-4x the file size. If you open in read-only mode, it's only a fraction of the file size (maybe 20-25% or so).
The site mentions 570MB file test, which is more than excel can handle. > 1 million lines.
Modern Excel uses a ZIP bag of XML, so it's pretty possible that a 570MiB .xlsx has way more that 1 million lines over several spreadsheets.