On the road in SFO... far away from BTV by way of EWR... so I'll have to wait to get the dimensions for you.

I have an older Ortlieb... I think it is the Ultimate 3? Not sure.
Love it for sure, just don't like the way it mounts.

The main part of the Acorn may be about the same size (seems bigger) - but you get 2 thin side pouches (energy food, thin cable lock, and reflective ankle bands), the 2 back pouches (camera, energy food, cell phone, etc), and the nice front pouch (brevet card, extra route sheet, gloves, and misc stuff). The Acorn is much easier to overload, as the top and give a little bit.