Sharepoint - How to change the layouts of the PowerApps-customized SharePoint form?

As of April 2018, PowerApps-customized SharePoint list forms do not support resizing.

Some poor workaround options:

• Create the full PowerApp. If you do that you will have to either jump from one URL to another between PowerApps and SharePoint.
• Use something that is not PowerApps. For example, SPFx

Update

Resizing is now possible with limitations

Just like any other Power App, form apps have a setting for resizing now. It's hard to miss:

Limitations:

There's actually a real easy way to increase the width on Powerapps forms that are integrated with sharepoint lists, so that not only is the empty whitespace utilized on the NewForm.aspx page, but on the list view itself if they click '+ New' that the pane will extend out further. Open the powerapps form you want to customize first, and then:

1. Click on File (top left hand side)
2. Click on App Settings
3. Click on Screen size + Orientation.
4. Then select the Orientation you want (Portrait or Landscape) and whether you want it small or large.
5. Make sure you click on Apply in the bottom right of the screen (it doesn’t really jump out at you).

Pick the landscape option, and then go back to your form and you'll be able to drag the fields out to the new wider width, then just save and publish.