Right now it's not possible to choose canvas sizes in Sketch, but in an upcoming update choosing a document size will be possible. Until then, hopefully this information will help a bit:
Exported files have a dimension of 2048 x 1536px (for anyone working on an iPad Pro it's 2732 x 2048px); if you're sending the work to Photoshop CC, you can resize the work up to (if I'm remembering correctly) four times without losing image quality.
Does that help?
This is helpful, thanks.
Please add 18" by 24" at 300dpi size to iPad Pro. And if you want to one up Procreate make a 24"x36" canvas as well . If anyone can do it Adobe can. Procreate has this and it is nice for posters . Even if it's iPad Pro only please!!!
Larger sizes are coming... in the next release of the app.
Hang in there.