I am planning to alter my bathroom who supploes the largest showers ?

Custom tile or manufactured enclosures?
Are you asking about shower bases nor complete enclosures? Can you be more specific? Regards, Tom

There are few big manufacturers that make pre-fab shower units. For regular (plain) showers you can go to : Lasco Bathware. For more sophisticated units I would recommend Ariel showers: Steam Showers Inc. - Steam Showers, Whirlpool Bathtubs, Steam Generators, Toilets, Shower Panels, Saunas, bidets, enclosures and Bathroom Vanities. See their web sites for sizes.