Treble,
It really depends on the mushrooms, lighting, etc. Most of mine started reproducing at about 1.5 - 2" in diameter. I have had some produce smaller in size than that though.
I have one type that showed up on my live rock and i paid a lot of attention to it at my LFS since it's what i have and those are less than 1/2 inch in diameter when they reproduced..guess it just depends on the type.