What app are you talking about?
If you mean the Remote Desktop app, you can resize it in several different ways. There is window mode and fullscreen mode, which can be toggled by clicking the appropriate icon on the bar beneath the screen. In Window mode, you can point the controller at the side or corner of the window until you see the white line around it, then grip and drag the window to resize it. In either mode, you can grip the window near the icon bar and pull it closer to you, or "push" it further away to make it bigger or smaller. To view your surroundings, you can click the passthough icon on the icon bar (it looks like a pair of goggles) or simply double-tap on either side of your headset (the white plastic arms) with your hand.
If you are talking about Link, then there will be a white-tiled grid all around you. That cannot be changed. There should be a console panel in front of you at knee height. You can also bring up a remote desktop here, similar to the one described above. If you can't see the console panel, try clicking (or maybe pressing and holding for 2 seconds) the menu button on the left controller. Tip: you can add more Remote Desktop screens by clicking the "+" button on the Link console panel.