Im new to the oil fired world...

What do you guys think about a 6" stack on a OIL fired 2x6?
I picked up a used leader rig that has a 6" stack. The collar on the arch is also 6" round, and not a oblong normal base stack.
I just ordered brand new 8" stainless stack (to have options). Should I somehow enlarge the collar/plate to be more draft friendly? Have a new plate made, or will the 6" collar (w/ 8" stack) be fine for oil fired?
Is a bigger diameter always better for oil fired too, or will you just lose heat faster?

I guess the bottom line question: is a 8" stack on a 6" round collar going to be ok?

Thanks in advance,