What is the size of a Scratch screen?

When you create a project in Scratch, the stage area is always 480 by 360 pixels. This is the visible area where your sprites and backdrops appear. If you are designing a backdrop for a Scratch project, you should create your image at this exact size, or at a multiple of it, so it fits perfectly without cropping or stretching.

If you want a larger visual display, you can present the Scratch project on a bigger screen, such as a projector or a large monitor, but the internal resolution stays the same. The image will be scaled up by the display, which may make it look slightly softer, but that is normal.

For a wedding, this is not about the software. The question might refer to a physical backdrop from a company that uses the name Scratch. If so, the size depends on the product you buy. Check the product description or contact the seller for exact dimensions. If you are making your own, you can choose any size that suits your venue, such as a 2 metre by 2 metre square or a wider rectangle.