How to do the layout for larger monitors and resolutions?

Hello!

Develop on a standard monitor 24" 1920*1080

Made another project for a client, protest - all is displayed correctly, centered etc.

But the client said that everything is very bad (although it already 100,500 project account before such claims were not)

After a few hours of removal of the brain was that this project should work on monitors larger than 24", in this case, he's got a 27" monitor. Although the design he gave me was actually calculated on the width of the 1440 monitors (I have successfully customized it and under 1920)

And really, having a second monitor is a 15" I slightly expanded the browser more than 1920 pixels wide and saw how everything goes

How to test for large monitors that had no such problems? I think my method with the extension Windows on the second monitor slightly curve and just not comfortable