I dont have dimensions handy, but as a father I can tell you things fit -snugly. I mean I'm comparing it to our Nissan Murano which has lots of room for kids and stuff. But in the A3 the rear facing seats require front seats to be fairly far forward, so you'll want it on the passenger side. Our full sized stroller (the type that has removable baby carrier that snaps in to the car seat base) doesn't fold up too far, but the handlebar flips up making it just fit in the trunk.
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So all in all I think it's designed for standard parenting needs, but I'm sure someone can locate a spec sheet somewhere. Otherwise I'd suggest to go visit your dealer
Ian