Having experience in selling merch and pins, I usually take the printing needs of clients in mind and do artworks in large resolutions.
You can choose a size to your likings, for example a 4x4in print, or an A4 paper size artwork. All of them will be drawn in 300 dpi, suitable for printing.
Since I use clip studio paint, I would give them the base file of .clip if they asked for it, but normally I would just give it to them in PNG formats in 350 dpi with different canvas sizes depending on the client on how they want it.
Mostly I do the artwork in A3 size which is 42 x 29.7 cm 300 dpi. I can do upsize until A1 but of course that will cost more. I provide PNG file mostly, transparent background PNG, and for PSD file I charge more.
What about the file format I usually provide such file formats as PNG and PSD.
About canvas size and resolution I usually use 300 dpi and canvas up to 3000 pixels, but if the client needs a specific illustration format I can customize according to the client’s requirements!