If you visit the Display Settings for your Mac, you'll find that you only have a handful of screen resolution options. For standard use, you'll get by with these. If you need a specific, unsupported resolution you'll need help from a 3rd party tool. There are many options out there. RDM is a free and open-source option.
Once you have it installed and have given it Accessibility permissions, open the menu from your top toolbar and select the resolution you are looking for.
I use RDM to adjust my screen resolution to 1280x720 for optimal screencasting.