Step 1: On the top menu bar, click “Workspace” then hover over “Primary Display” then click on your 2nd screen (which may be named something like “HDMI-0”.
Step 2: Again click on “Workspace” and then click on “Reset UI Layout”. Now quit and then re-launch Davinci Resolve.
If you try to open a project in Davinci and the UI is too large (overflowing off the screen) or too small (not filling the screen), then go to step 3.
Step 3: On the top left menu bar, click “DaVinci Resolve” and then click on “Preferences”. Click on the “User” tab at the top of the popup window. Click on “UI Settings” in the side menu. Go down to “UI Display Scale” and choose a smaller or larger %. Click Save. Quit and then re-launch Davinci Resolve.
Note: Unfortunately, Davinci Resolve can be a bit glitchy, so you may have to repeat one or more of these steps, but they will eventually get you what you want (a full-screen view of Davinci Resolve on your second monitor).