In order to create Custom Ad Sizes, navigate to WordPress Dashboard > WPAdCenter > Custom Ad Sizes.
Click on the + Add Size button to add a new size. Click on the Edit button, which will allow you to edit the dimensions and name of your choice.
Once you are done, click on the Save Size button to save the newly created size.
You can find the custom size you are looking for by entering the name, width or height in the topmost search box which is labeled as Filter.
You can create a custom size with width between 1px to 1920px and height between 1px to 1080px. You can find your added custom ad sizes with other sizes under the Custom submenu under Ad Sizes. as shown in the image below.