A 3size document in Aftereffect

Thanks so much for your assistance into the matter Gerard and Martin

warmest

As MartinR said, you have to use the appropriate pixel dimensions. There are no inches in web images or video. To figure out how many pixels are in an A3 size document determine what PPI (pixels per inch) you want to use when you print the document then create an A3 document in Photoshop and make sure the PPI matches your printing requirements, then look at the number of pixels. 300 DPI is probably higher than it needs to be but the optimal size depends entirely on the kind of image. You'll probably want to render a PSD or a TIFF from the Comp and have the project set to at least 16 bit if the quality is going to be your biggest concern. Choosing the right format for the render is at least as important as choosing the right pixel dimensions.

I created such a file with PS and 300dpi (standard print resolution) and it turns out as 3508x4963 composition.

Try to even increase this size, just to be sure to get a good resolution in the end.

*Martin