I recently wrote up my new raspberry pi workstation with a nice new ViewSonic VA2246M but by default the Pi would only output 1600×1200. The configuration for the HDMI lives in the config.txt file in /boot. I had to work through a few different hdmi_group and hdmi_mode settings to find the right combination. I found a page that outlined all the possible combinations and I set about finding which one worked for my setup.
To save you an hour or so you’ll need to edit the /boot/config.txt file on the pi.
You’ll need to then find the section that has the following two lines
#hdmi_group 1 #hdmi_mode 5
Remove the comments and set the following
hdmi_group 2 hdmi_mode 83