To use it, you will need the very latest version of MSE, otherwise it will pester you about card stamps. The best way to update is to go to the following link, download the Lite pack, unzip it into your MSE folder, allow overwrite, then reload MSE: magicseteditor.boards.net/page/downloads
You can get the template at the normal MSE resolution of 375x523, or the WotC image gallery resolution of 646x902, which will look better, but will not align with the other cards in your set if you export them as a PNG. It will have the correct size if you export as a PDF though. If the card is too big to edit in the editor, you can change the zoom value by going in the edit menu > preferences > display tab.