Interesting question - and a bit of a semantic dilemma.
IMO, the real answer to the question as asked is 'no', there is no way to determine "choke size" w/o knowing bore size. By "size", I'm assuming you mean a decimal figure that could then be compared to a choke chart to see where your figure falls.
However, as pointed out correctly above by Gregsy, you can take a gun and a load and head out to a patterning board. There, if all is done correctly, you can determine the pattern percentages for such load(s) @ whatever range you choose to test for.
Is such a % figure the "choke size" for the barrel(s) tested? IMO, no, though you've learned a lot more about that gun w/ that load than any bore minus muzzle decimal figure alone could ever tell you...
