Hi Steve. Over here in the UK I've had my 5" Batch Box lit for around 40 minutes and recently added a 6" diameter very dry ash log to the fire. It's burning well and I could probably fit a few more in the firebox. Your 'large' pieces should work fine - provided they're very dry of course
In my particular BB (5" sidewinder) it seems to perform best when using different sizes of fuel and also a mix of hard and soft wood. We've mainly got oak, ash and sycamore but larch and sitka spruce burn very well in the system. Broken pallet planks work well, especially when mixed with the hardwood.
Yes Steve - 5" refers to the diameter of the riser. All the other measurements are in proportion, see Peters Batchrocket.eu site Batch Box Dimensions