I thought about a large tile for my master bath, but my designer said we would have to change the plumbing to use a trench drain. And it couldn't be a shiny tile, or it would be slippery.

I have a shower with a large format (18 X 18), very slippery tile on the walls, so I went with the same tile in a 2 X 2 for the floor so that the grout lines would compensate. I also have another shower with 12 X 12 tiles on the floor that is smooth, but completely non-slippery. So my advice to you is to determine how slippery those tiles are: Coefficient of Friction for Tiles. Also be aware that, if you have a conventional center drain, it will be a challenge installing 5 X 5 tiles as they taper in to the drain.

Lastly, I am going to ask you if your tile looks like marble, or actually is marble? If it's natural stone you may want to reconsider. There are numerous threads on GardenWeb by people who have had bad experiences with staining and an uneven look when they installed marble on the shower floor. Some of these are worth reading: Marble tile staining

We would never recommend/design a shower floor with that size. Two reasons, both safety (slippage) and achieving a proper pitch (possible but not an easy task and you will need to cut the tile into smaller shaped pieces nearer the drain, like diagonal, depending on linear verses standard drain). Now if you have a very large shower, the pitch is easier. I would be explaining all of this to our client if they requested this tile and be clear as to how proper pitch would be achieved and the aesthetic outcome (and cost of labor) as well as making it clear that slippage is a greater issue with this size.

Dragonfly-

I know that you are a professional in this area, so I am not trying to pick a fight, but I do want to point out to others that there are some larger tiles that are non-slippery, even when one has soap underfoot. Mine are 12 X 12 porcelain tiles that have a slightly-striated texture that has never once been remotely unsafe. I am not representing that this is the majority of tiles, but you can find them.

I went with a linear drain across the center of the shower so that the shower floor could slope from each end in two flat planes:

kudzu9 as we said, "depending on linear verses standard drain", this can work and you are correct if it works in your specific situation (we've used that same approach when appropriate), but the size of the shower was not revealed here and is very important for proper pitch. Just remember, we are speaking to the masses on these forums and we would be less than responsible to suggest it is the correct approach for most of the designs and materials we see presented here.

Corrector-

Please explain what you mean. In my experience, if you are not using a linear drain, and you are using large format tile, the only way to get a smooth transition that follows the slope to the drain is to make cuts in some of the tiles so they can be "bent" to conform to the slope. Tiles are planar (flat), while a conventional shower floor is not. I have seen some floors that were done with a conventional center drain and large format tiles and it just looked "off."

You can do an X cut to the center drain, creating 4 individual planes. You could technically use just 4 pieces of stone or tile for that, if it met the DCOF rating.

" In my experience, if you are not using a linear drain, and you are using large format tile, the only way to get the proper slope to the drain is to cut the tiles so they can be "bent" to conform to the slope "

[https://www.houzz.com/discussions/using-diagonal-cuts-to-slope-your-shower-floor-planning-guide-dsvw-vd~635709[(https://www.houzz.com/discussions/using-diagonal-cuts-to-slope-your-shower-floor-planning-guide-dsvw-vd~635709)

We aren't disagreeing. I didn't say it couldn't be done at all. My post was so that people understood you couldn't lay large format tiles on such a floor without cutting them in certain ways. The link you posted illustrates that point.