Got some measurements.
Stock battery:
L: 238 mm
W: 127 mm
H: ??? (can't accurately measure because of the battery tray. I measured 190mm, but group 51 is meant to be 227mm tall)

These are off maybe by 2-5mm because I didn't remove the plastic shroud that covers the battery.

I tried measuring the largest battery I can get in, but it's not that accurate. I got:
L: 280 mm
W: 154 mm (with stock snorkel in front of battery. Larger without)

So if I take off my stock snorkel, I'll be able to fit in a D34 bluetop battery!!