If you want to set a custom resolution on your Mac, you can do so by going to the Displays preference pane in System Preferences. From there, click on the Scaled option and then select the resolution you want to use from the list of available resolutions.
If the resolution you want to use isn't listed, you can click on the Custom button and enter your desired resolution into the boxes provided. Once you've made your selection, just click on the OK button and your new resolution will be applied.