i am currently trying to figure out the best way to feed a 100amp panel fed from a 200amp Main that is 300 feet away from each other. Do you have any suggestions? and yes i know its going to be expensive.
The minimum legal size for 100A is #1 aluminum, however, voltage drop will be almost 5% with that wire.
2/0 aluminum is at a pricing and availability sweet-spot and is probably just right for that distance.
Voltage drop is proportional to actual load so it matters what the loads actually are.
You're supposed to breaker the circuit for 125% of the expected load, so a 100A breaker implies you expect <=80A of load. At "full" 80A load your voltage drop will be 3.25% @ 300' w/ 2/0 aluminum wire, which is perfectly fine. $2.50/foot so $750 whole job.
The minimum aluminum size you must use is #1, and #2 is legal if you downsize to 90A breaker... but there's not much cost savings to downsize wire. Even #2 is $1.68/foot.
Aluminum wire is the preferred wire for heavy feeders of this sort. Disregard anything you may have heard about aluminum wire for small 15/20A branch circuits; that is not applicable here.
If you really are hell-bent on superstition, you can pay $14/foot ($4200 whole job) for #1 copper, which will give you 2.99% voltage drop at 80A.