After doing a little googling I was unable to find a way to show the current viewport resolution within the UE4 interface, but it is possible to capture what you can currently see in the viewport, at the resolution you are currently using through the shortcut key F9. You will see a popup detailing the location of where the screenshot was saved to, and clicking the location will automatically open it in Windows Explorer.
By looking at the details of the images you will be able to see the dimensions of the image captured. If it is not to the size you require then adjust the panels surrounding the viewport in UE4 and capture a new screenshot. Repeat this until the screenshot is captured at the resolution you require.
Once you have the layout you desire, it should now be saved for future, but to ensure that your layout is saved go to Window > Save Layout.