How to figure out render output size?

A couple weeks ago, I used batch render instead of backburner because of network issue at my work place. When I opened a menu, I noticed a tab below that was called ‘Print Size Assistant’. I never used it but it was a handy tool.
Go to Rendering > Print Size Assistant. You can change a paper size and DPI easily and then when you are okay with settings click Render Setup. It will automatically change Render Output Size for you.