Maximum allowed file size
The maximum allowed size of a single file.
The total of all maximum allowed file sizes for all your upload fields should be less than your PHP value
The maximum of a single file upload should not exceed the PHP setting
If you are attaching the file to the notification email, you should make sure that your PHP setting
memory_limit is about 8-10 times the maximum file size.
If you are not attaching the file to the email, but storing it on the server instead you can probably safely upload a file much larger than this. If in doubt, upload a test file at your maximum limit and see if the form works correctly.