5x5 plenty big for two - i find even 4x4 feels roomy. 5x5 would leave room for a bench seat along the back wall, if you put two showerheads on the opposite wall (or one large showerhead in front and/or above). I much prefer the side-by-side arrangement to one showerhead each on opposing walls, which usually leaves you showering back to back rather than alongside each other.
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