In this document, we develop a method to dimension antagonistic artificial muscles to actuate a joint while modulating its stiffness through the biological principle of co-contraction. Our method relies on a linear model of the force-length characteristic of biological muscles. This model is then used to derive relationships between the activation levels of the artificial muscles and joint torque, equilibrium angle, and stiffness. In particular, we provide a graphical representation of these relationships that illustrates the principle of co-contraction and underlies our dimensioning method. Then, we illustrate our method with a numerical example by showing how to find the main characteristics of artificial muscles for a given operating point and stiffness range.