I've had a few experiences where a girl was surprised by my size (or lack of). One of my favorites was a girl I met through work. We'd see each other every now and then and shoot the breeze when there was down time. Eventually we decided to go on an afternoon hike on a day off. This particular hike was known for its hot springs and a nudist population who enjoyed frequenting them. I mentioned this to her and this led to several flirtations messages between us about enjoying the sun, hot springs, and sans clothing.  

 The day came and we reached the hot springs.

She and I exchanged glances and she said "what are you waiting for? you can't go into the hot spring with clothes on." 

"I'll strip if you do,"  I replied.

"Ok, deal," she said playfully with a look of anticipation on her face.

I turned around so my back was to her (I wanted to take in her response when she saw my penis). I stripped my shirt, pants and undies off, tossed them aside near the hot spring and turned around.

She glanced down and immediately started laughing.

"What's so funny?" I asked

"Nothing," she said, covering her mouth trying desparately to contain the laughter, but unable. 

I knew what was so funny. She took one look at my 3 inches soft and couldn't contain herself.