The maximum size per file that you can upload is 50 Meg which is very large. If the file you are attempting to send is larger than 50 Meg in size it will take a long time to upload, even if you have a high speed connection, and then it will fail for exceeding the allowable limit.
If the document you are trying to fax has been scanned it should not exceed 2 to 5 Mb in size. If your file size is larger than that, than we recommend you review the question on Poor quality faxes from scanned documents
If you continue to encounter difficulties, please start a Trouble Ticket and we will be happy to assist you.