So this sounds nuts - but really making your art ready is simple.
Always simpler if you plan a bit ahead and create your work to scale.
So create your art preferably from the start at the biggest size you want!
Because it's much easier to scale down than up.
If you create an artwork that is smaller than you want the finished piece to be you risk the integrity of details during the enlargement process.
Much better just to work at the size you want to print.
2 KINDS OF ART
Vector files (Illustrator / CorelDraw) are great so long as you turn all fonts to outlines.
Pixel files (Photoshop or similar) should be 200 - 300dpi
Art should be supplied at the size you want it.
That size should make sense in your screen and on your finished piece.
Save and send your work as a PDF