Photo size

More details please-

Your App and version number?

What do you expect to see "full size"?

You can hide the Filmstrip at base of Lightroom-desktop v8 UI.

You can hide the Photos/Albums lists panel on the left of screen.

But when editing in v8 you would obviously need the Editing panel open with the adjusting sliders.

https://community.adobe.com/t5/lightroom-ecosystem-cloud-based-discussions/does-lightroom-desktop-v-8-0-have-a-full-screen-mode/td-p/15012277