Each individual file that you upload must not exceed 15MB. When you are bulk uploading, you can upload many files together, each with a maximum of 15MB per file.
You may want to keep in mind however that once your total file manager storage exceeds 50 MB, additional storage charges will apply.
Another useful thing to know about the file manager, is that you can set it to automatically resize large images when you upload them. Under the 'Advanced Settings' options you can see the radio button options to do this.