Hi Josh,

Interesting question. Two years ago I would have replied non-dimensional scaling is the way to go, but I am now a bit more uncertain.
If you have a sizable sample with good variability within the sample for these covariates (sex and size), I would probably run a multiple regression to investigate any relationship, and then residualise to remove the statistical effect of the covariates.
I would still perform non-dimensional scaling according to At Hof 1996 to have a look, and check if there is a residual relationship with size, because it could be interesting to (try to) find out why.

Good luck,