Hello,
in my experience with print media you should not go for the full resolution by 300dpi which is about 9933px x 7015px. This would be pretty unhandy for Sketchbook, needs a lot of performance and is usually not necessary.
For a detailed artwork I would recommend you to go use A3 300dpi, which should be about 4960 x 3500 pixels. For some comic art drawings/paintings 3500 x 2480 pixels should be enough.
A1 poster prints arn't my daily work, but I 've got goor experience with 4960 x 3500 pixels, even for photographie.
Depending on the kind of art you want to create, you might think about switching the software to something vectobased like Adobe Illustrator of Affinity Designer. Then you don't have to care about size.
