I am trying to analyze some data and keep going in circles about the best test to use. I have fish data from 2 sites and am looking to see if there is a difference in some hormone concentrations (continuous) between the sites. However, it turned out size of the fish differed between sites so I also want to include this in my model, leaving me with one categorical independent variable (2 levels) and one continuous independent variable.

So far I have made linear models and used a multiple linear regression but now I'm wondering if that is the right thing to do when one of the independent variables is categorical?