Is there a service or program in which you can set the resolution, diagonal, and see the result on the monitor?

Question

Screen resolution

Google and Yandex for half an hour did not find anything except mobile device testers with some shit from apple in the templates. I need an extremely simple and convenient program or service in which you can set the desired resolution, set the diagonal and look at your monitor in this size and resolution it will look like a video site, etc., it doesn’t matter (although it’s preferable, of course, that there are sites, because maybe someone else won’t be able to find a similar service, but they will stumble upon this question and hopefully the answer is in it) I don’t test any sites, designs, etc. (at the moment) I just want to choose between tablet versions in favor of performance / quality and I would like to see how different resolutions and content in them look on a certain diagonal

Answer 1

responsivetest.net
beta.screenqueri.es
quirktools.com/screenfly

Answer 2

Firefox in developer tools is able.