We present a method to derive sizes of marginally resolved star clusters from HST/WFPC2 observations by fitting King models to observations .
We describe results on both simulated images and on observations of young compact clusters in NGC 3597 and NGC 1275 .
From the simulations , we find that we can measure King model concentrations ( c ) to an accuracy of about a factor of two for all combinations of c and King radius ( r _ { 0 } ) of interest if the data have high S/N ( \buildrel > \over { \sim } 500 for the integrated brightness ) .
If the concentration is accurately measured , we can measure the King radius accurately .
For lower S/N , marginally resolved King profiles suffer from a degeneracy ; different values of the concentration give different r _ { 0 } but have comparable reduced { \chi } ^ { 2 } values .
In this case , neither the core radius nor the concentration can be constrained individually , but the half-light radius can be recovered accurately .

In NGC 3597 , we can only differentiate between concentrations for the very brightest clusters ; these suggest a concentration of \sim 2 .
Assuming a concentration of 2 for the rest of the objects , we find an average King radius for the clusters in NGC 3597 of 0.7 pc , while the clusters in NGC 1275 have an average radius of 1.1 pc .
These are similar to the average core radii for Galactic globular clusters , 0.92 pc .
We find average half-light radii of 5.4 pc and 6.2 pc for the young clusters in NGC 3597 and NGC 1275 , respectively , while the average half-light radii of Galactic globulars is 3.4 pc .
The spread in the derived radial parameters in each cluster system is comparable to that observed in the Galactic globular cluster system .