Welcome, Electros. Please give us a first name to use.
Don't know where you woulda seen 51x70 inches billed as a minimum size for a walk-in shower. That's actually a rather roomy shower by most residential standards.
How much room you got? Plenty showers out there in the 36x36 range. Not gonna be doorless at that size unless you can tolerate a lotta water on the floor outside.
My opinion; worth price charged.