The DCOF has to work for a wet area. It has to have grip for wet slippery feet. Which does make it a bit harder to clean. If that DCOF is fine? Then you could use one of the super large format tiles, and just have one single tile on your floor.

More practically , a clients budget is what limits this. Large format tile, and the required floor prep, plus the plumbing moving for a trench drain, is all MUCH more expensive than a standard small tile and bowl shape. You see the walls much more than you do a shower floor, after all. Not many want a 20K top designer look shower when they can get “good enough” mid price look at 10K.

With a linear drain and a single slope, you can use any size you want...although you should have a well-textured tile for slip resistance. On a traditional, a 2x2 is generally used. If it is a bit larger, I have gone as big as a 4x4....but not much more. Just remember this, you are tiling a bowel......

My 5' X 7' shower has a linear drain and 12" X 12" tiles. I chose porcelain tiles with plenty of slip resistance and it's very safe, even with soapy feet. And I love the fact that there are very few grout lines to try to keep clean.

"Our contractor and tile person and tile shop salesperson are all advising against it"

All three need to educate themselves. Which should make you a bit concerned about who you hired. If you are going curbless (aka barrier free) you should be very concerned.

As stated above. Any size. It's all about the DCOF.

After reading this link you will have more knowledge than the ones giving you advise.

https://www.tcnatile.com/images/pdfs/DCOFBrochure_Aug2013_Comp.pdf

You may wish to read this one also.

https://www.ceramictilefoundation.org/homeowners-guide-to-hiring-qualified-tile-installer