As the others have rightfully pointed out, there are quite a few variables involved in determining how much cooling is necessary for your SSR. In order to determine this with any level of accuracy we need some additional information from you, such as - SSR part # & manufacturer, full load current, ambient temperature inside the panel where you will mount the SSR, warmup period and duty cycle (time it takes to reach setpoint temperature and the on-time / off-time while the temperature is being maintained).
There are a few articles here on calculating SCR temperature and heat sink requirements that may prove helpful.
Heat Sink Calculations - http://www.crydom.com/en/Tech/Newsletters/Solid%20Statements%20-%20Heat%20Sink%20Calculations.pdf
SCR Die Temperature Calculations - http://www.crydom.com/en/Tech/Newsletters/Solid%20Statements%20-%20Estimating%20SCR%20Temperature.pdf