I'd guess that this is a good sized buck. As a general rule, I've heard the higher up on the tree the rub is located, the larger the buck
This is also what I remember reading, possibly from D&DH. I thought it was something like if it is 24" or so and lower to the ground it is a smaller buck but if it is 24"-30" or more it is a larger buck. I also remember reading that the diameter of the tree being rubbed does not have anything to do with the size of the buck. Of course I could be wrong but that is what I remember. I don't know where you are from but a LOT of deer in MN are already dropping their antlers, which is pretty early for that to happen. I think typically it occurs in February to March here in MN.
Good luck looking for shed.
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