You can set a limit in the webserver procedure, but other than that there is no limit.
The key limit in play is the number of these large files you may receive smiltaneously. They are stored in ram as they arrive, and the server can only use a finite amount of ram (around 1 to 3 gigs).
Compressing the large XML file before uploading it won't solve the ram problem ( assuming you need to decompress it at some point) but it will make transfer a lot faster.
Cheers
Bruce