Desktop screen goes smaller after Mac wakes from sleep mode since upgrade to El Capitan

I have a HP Pavillion 22xi monitor with my Mac mini and had no trouble with it untill i upgraded to El Capitan.my screen is full but when my mac goes into sleep mode my screen goes smaller,The ponly way i can get it to fill the full screen is a restart or use the underscan feature but if I alter that when i reboot the destop is to big for the screeen . Help