I've got about 1250W of power going through three SSR's in a reflow oven. I did not do the engineering on the heatsink but I trust the guy that did. The heatsink is just an aluminum plate a little bigger than the relays themselves.
Working out heatsink sizes is not hard if you know how many watts the device will dissipate. Find the degrees-per-watt rating for the heatsink and multiply by the number of watts. Then add the ambient air temperature. That will tell you how hot the back of the device will get. Of course the 'junction' inside the device will be hotter but the datasheets usually give you that information too.