Product review (5 stars): Works GREAT in my car for my potty trained twin toddlers
Works great for my newly potty trained 2.5 year old twins.  If we're out running errands, after a couple stores, I ask if they need to use the potty.  I have a Toyota Avalon with 2 cars seats in the back, so space is limited.  I sit in the back seat, put the potty at my feet, place my little one in front of the potty, she sits back and voila!  It's convenient and they seem to even have fun w/the whole experience.  I tie up the bag -- never had a spill or drip.

I'll admit, I was skeptical when I opened it, b/c it's super compact, but I guess that's what you want.  Small or not, it works great.  I'd definitely recommend this, especially to parents who balk at the thought of bringing toddlers (who touch everything!) into a public restroom.