No hard limits are available, it’s 100% up to LibreOffice, how it handles specific complex file content. Even a 20 MB plain text file can take 5 minutes to convert because it produces ~10,000 PDF pages.
So it depends on file layout complexity + a number of pages in resulting PDF.
The only way around huge files I found so far — splitting. Text & Office files can be easily split before converting them to PDF. Thankfully, there are many libraries for common file formats.