need help with new monitor in dual display

hello everyone

i have just picked up a new 23" monitor to go with my other 23" monitor both are 1080p. i am using them with a HD 7770 1GB. one is connected via dvi and the other is connected via HDMI. the monitor using HDmi is a computer monitor and not a TV and find when connected to HDMI it is not full screen yet the other monitor is. i know it is not the monitor because it is going full screen in BIOS. anyone know how i can make this monitor full screen. the new monirot does not have a DVI port and only other connection is the older VGA