Sorry for being Mr Thickie Blackadder Thickie here but.... surely it is the responsibility of BRCA/EFRA/EB or whomever to, with the help of their representatives, create a set of rules that will create a level playing field and be simple to scrutineer and abide by - hence they should be concerned with nominal voltage, capacity, even maximum current discharge, charging methodology and the homologation process.
Standardisation of size will be a benefit to many racers, but thats really up to the manufacturers to agree amongst themselves? If you desgn a chassis, you are wise to design ones that will accomodate as many cell types as possible and conversely, if you design cells then you are wise to design them so they fit as many chassis' as possible?? I see no reson why dimensions should play a part when output and capacity are the factors that affect its perfromance??
A 20c 5000mah 7.4v lipo shaped like a banana is no better or worse than the trakpower or orion equivalent, but people wont use it because its stupid. I would suggest setting the key technical specifications and leave the ones that are purely academic to market forces. LxWxH and ground clearance for the overall cars fine, but dimensions for the internals seems a bit unnecessary to me. But like I say, apologies if I'm being thick.
btw I have a zx-5 and orion carbon lipos

but I dont race right now anyway