This is from a post a few years ago:

As far as counterweights hitting the piston at the bottom of the stroke, the typical problem is the piston pin boss, ie metal below the piston pin. This thickness varies a lot from one piston design to the next. My Ross 99496 pistons have 0.150", Wiseco K296A35 have 0.220", my Venolias have 0.180" (see attachment). Everything must be checked at assembly for clearance, but this is the math:

CWmax = Counter Weight max dia
Pthick= Piston thickness below the pin
Pin = Piston pin dia
Rod = Rod Length

CWmax = 2(Rod - Stroke/2 - Pin/2 - Pthick - Clearance)
For example:

CWmax = 2(6.535 - 4.25/2 - 0.990/2 - 0.180 - 0.075)
CWmax = 2(6.535 - 2.125 - 0.495 - 0.180 - 0.075 )
CWmax = 7.32" dia

That happens to be the dia of the "cam cut" on a Callies 4.25" stroke crank counterweight for a BB Mopar.

With a 6.535" long rod, 4.500" stroke and assuming the Ross 0.150" and 0.060 clearance:
CWmax = 2(6.535 - 4.500/2 - 0.990/2 - 0.150 - 0.060)
CWmax = 2(6.535 - 2.250 - 0.495 - 0.150 - 0.060)
CWmax = 7.16" dia

That should work depending on the bobweight. I cut my Callies 4.250" down to 7.14" with a 2185 gram bobweight.

But with a 6.385" long rod and assuming the Ross 0.150" and 0.060 clearance:
CWmax = 2(6.385 - 4.25/2 - 0.990/2 - 0.150 - 0.060)
CWmax = 2(6.385 - 2.125 - 0.495 - 0.150 - 0.060)
CWmax = 7.11" dia

The rest of the counterweight can be larger since it isn't at BDC. Balancing will need to be addressed with those counterweights. It might work, but I like the 6.535" rod for that.
My Callies was cam cut from the factory at 7.55"/7.32", but I cut it down to 7.14" dia (round) and balanced at 2185 gram bobweight. The balance shop went between the lathe and the balancer to get this close, don't just use my dia with other cranks and bobweights.