Since this ebike is commercially available, there are lots of web links to it. Here's one with a decent photo: http://www.electric-bikes.com/bikes/electrec.html. (Note: it is currently called the "Rayos.")

The internal space in the battery box is the size of two 12V 12Ah batteries end to end, or about 12" x 3.75" x 4".

As an update, I contacted the helpful folks at Electric Rider since I noticed they finally found a good lithium battery for their systems. They do not currently have a fit for my bike, but may consider such in the near future. Based on what they are selling, I'm guessing mine would be no cheaper than $1000. They are using LiMn rated at 800 cycles. I would hit 800 cycles in about 4 years. I could buy a new set of SLA batteries every year, whether I needed them or not, and spend less money. Am I missing something, or does it seem like lithium is not quite worth it for a 24V system?

How much capacity do I need? Based on my worst case scenario, I think I would be safe with 24V 10Ah.