Try increasing these settings in the php.ini, the default size ( typically ) is 8MB
; Maximum allowed size for uploaded files. upload_max_filesize = 40M; ; Must be greater than or equal to upload_max_filesize post_max_size = 40M;
And after that you have to restart your server to apply these settings.
What this means is that PHP is limiting your file upload size to these 2 settings. You may not need 40M but you should increase it to maybe something like 120% of the largest file you want to upload, which will give you a bit of breathing room.
This is typically the first thing I configure on my servers and I usually do about 80MB, anything more then that and it's just easier for my clients to use sFTP.