I don't really know anything about this, so please excuse my stupid questions.

I want to create a document in Krita that I will send to a printing company to be printed as a poster. The size of the final print will be 18 inches by 24 inches. So, how should I set up the document. I think I should set the color mode to CMYK and the dpi to 300, but that's about all I know. What should all the other settings be set to ?

Also, what size should I make the document ? Does the document in Krita actually have to be 18" x 24". When I try a document that size my Krita starts lagging pretty badly, especially if I'm using a larger brush (I've already tried changing the memory settings, open GL, and unchecking texture buffer. None of those helped at all. Setting brush spacing to auto helps a little, but not much ). With an 18" x 24" document, everything seems to slow down, it takes a long time for any action to complete. It makes Krita almost unusable. If I made the Krita document half-sized (9" x 12") or even one-fourth sized (4.5" x 6") when I set up the document and then resize it to full size (18" x 24") before I export it, would that have a negative effect on the quality of the final print ?

Finally, unless the printing company specifies a certain file type, what is the best file type to export the document in ?

I appreciate any help that anybody can give me.

Thanks...