As 6x6 says, spacebar gets you the full sized preview. Likewise, spacebar also lets you view files from the Finder in QuickLook. (Oops, 6x6 already said that while I was typing!)
If you use Photos in Full Screen mode, then you can turn the sidebar off, and have previews take the entire screen. The shortcut for full screen, it's fn-F. Well I'm pretty sure that's standard. I can't remember if I just set that myself. The thing is, you can set the shortcuts you want in System Settings at Keyboard Shortcuts>App Shortcuts.
Create keyboard shortcuts for apps on Mac - Apple Support
You can use that setting to change already existing shortcuts, as well.
There are several apps that could use some extra shortcuts, don't you think?