Not sure how you are proceeding. Also could not find suitable info about Web monitor sizes (http://www.w3schools.com/browsers/browsers_display.asp
I am a Photoshop and Illustrator user. Don't use image programs to create Web pages so do not understand your approach. A simple pencil sketch (1 column or 3 columns, etc.) works fine.
Think "aspect ratio" might be more appropriate than pixels. Seems you are aiming for a "landscape" desktop computer monitor. What about portable devices and phones, some of which can be "portrait" oriented? Viewers (like me) will likely change screen resolutions to suit themselves.
Seems like you are going for a "fixed" layout instead of fluid and/or responsive
Cannot, at the moment. imd good reference links for you.