I wouldn't recommend it. Cats have a natural instinct to bury their waste, so a litter box best fits their needs. And though it "might" be acceptable when its tiny, you certainly don't want a full grown cat peeing and pooping in your shower.
Congrats on your soon to be adoption of a new kitten. I'll post links to articles that might have some helpful info for you.
Are you talking about actually using the shower itself? If so, see above! If you are talking about placing the box in an unused shower to make cleanups of kicked litter easier, I don't really suggest that, either. The drains are really not designed for that, and you could easily cause a serious plumbing issue.