Thanks for your post @danchircop and I'm sorry to hear you're experiencing this issue. I checked around the PowerApps Community and found this response from a member of our Product Team in regards to the PowerApps Desktop version. Take a look and let me know if it helps.
When building apps to be run in the browser exclusively, I would recommend that you select the "Tablet Format". This format is designed to work well on desktops, tablets, and even large phone screens. It is on our roadmap to further enhance this experience so that you may create apps from data using the Tablet format, however I don't have any specific dates to share yet.
Please feel confident that any app you build in tablet mode will work well in a browser / desktop view in the meanwhile. We have a great selection of templates you can use to get started building beatiful apps for destop and browser. You can locate these at web.powerapps.com on the Home page. Click on the Office filter in the templates section to see some of my favorites.
Please review these articles for additional guidance:
https://docs.microsoft.com/powerapps/maker/canvas-apps/set-aspect-ratio-portrait-landscape
https://docs.microsoft.com/powerapps/maker/canvas-apps/get-started-test-drive
We will continue to enhance formatting options, and have fully responsive design available now in Model Driven Apps, with roadmap plans to include fully responsive app design in canvas as well.
More on Model Driven Apps:
https://docs.microsoft.com/powerapps/maker/model-driven-apps/model-driven-app-overview
@Anonymous