Issues with macbook and external 1080p monitor

Hey, i recently got a BenQ G2420HD monitor and connected it to my macbook via mini displayport to hdmi cable. i set the screen resolution in default to 1080p, but i have a small issue with the resolution. part of the picture is being cut off at the top,bottom,and sides of the external monitor. this can be easily noticed by having the same background picture as on my macbook screen. since the connection is through hdmi i cannot adjust the picture size on my monitor, what can i do to fix this?