beaten at the post !

This is the calculation I did before I saw Jame's post:

Using my Basal Metabolic Rate (as calculated by: http://health.discovery.com/centers/heart/basal/basal.html ) I get a value for k of 3.6. Taking solar energy flux to be 1370 Wm^-2 and rho equal to that of water (1000kgm^-3) I calculate r to be about 28m. Which corresponds to a mass of 3x10^7 Kg (or if you prefer Blue whale units) 140 Blue whales.

(Note if I use k=5 I get the same results as James. Damn you James for posting the solution before me!)