I'm in the same boat; I've got a K40 that's in pieces pending installation of my Grbl-based controller. It's going to need a cooling solution...
You can measure the maximum heat you need to dissipate by measuring the electrical power the machine pulls from the wall when the power output to the tube is turned up to the maximum. For making the measurement; I'd just temporally rig up a bucket with some ice water in it. One of those "kill-a-watt" meters comes in handy here if you don't have a clamp-on current meter or a convenient place to clamp it on.
There will be some power that's pulled that never makes it to the laser tube which goes into inefficiency of the electronics; as well as some that goes into the optics transport motors; but this should give a ballpark figure to shoot for when sizing the radiators. If you have "too much" cooling capacity; you will just have better performance out of your laser; if you have too little, you'll cook your tube - so shooting high is a better idea than shooting low.