Radiating heat into space is done through the age old mechanism of radiators. The Space shuttle had a pair of radiator panels built into the cargo bay doors (hence the doors were always open once in orbit) and the ISS has some pretty impressive sized radiators (although much smaller than the rather spectacular solar array)
ISS showing deployed solar arrays and radiators
For ordinary spacecraft, cargo ships and even military vessels on a normal cruise, this will suffice. Calculating how much radiator area you need can are done using formulas provided in the ever helpful Atomic Rockets site here. There is no sort of general answer, since radiator area will have to be calculated on the basis of things like what sorts of systems are being used, how large is your crew and so on.
The reason for using heat sinks rather than radiators would come from non standard situations. Military spaceships deploying for battle might prefer to pull in their radiators to prevent them from being shot off the ship. At the same time, they are also ramping up high energy equipment like lasers, which will generate a lot of waste heat (once again, vary depending on the type of laser being used). All this heat needs to go somewhere, so heat sinks are the way to go. For civilian spaceships, aerobraking at interplanetary speeds will put a lot of stress on the heat shields, but once again, the conditions don't allow for a deployed radiator to be used while aerobraking, so a heat sink would be appropriate for these kinds of ships as well.
Once again, there is no "one size fits all" equation, since you need to know how much heat needs to be absorbed, what time frame you have (both to absorb heat and if required, eject it through the redeployed radiators) and even what materials you are using in your heat sink. A very low efficiency, low tech heat sink could be made simply by piling rocks in a large cavity and pumping hot coolant inside (some old solar thermal home heating systems did exactly that), while other materials like phase change salts or even large tanks of water or hydrogen could also be used, with vastly different efficiency levels.
One thing which you should consider is that for most spacecraft in a plausible mid future, the "Tyranny or the Rocket equation" will control spacecraft design. While the equations are relatively easy to do, the consequences can be summarized in the expression "every gram counts". Extra mass devoted to heat sinks means either the performance of the ship takes a hit, or you must build an even large ship, in an escalating spiral. Ejectable heat sinks have a certain 'flair", but the extra mass of coolant piping, couplers, valves, quick disconnects and the shaft(s) to eject the heat sink itself will have a very negative effect on ships performance.
What you really want to do is be able to repurpose existing systems to help you out. Fortunately, you can do this!
If your ship is powered by a nuclear or fusion reactor, then in addition to the nuclear fuel there will be a lot of reaction mass. Hydrogen is the best in terms of performance, but ships may choose to use water since it is cheap and easily available throughout the solar system. Since the water needs to be liquid, there will already be a system of heat pipes running through the water tanks, and the tanks have all the associated plumbing needed to fill and drain them. During a space battle, the heat is being shunted into the water tank, and the water is gradually absorbing the heat. You can keep adding more heat until the water starts to boil (and if you are willing to absorb the mass hit, you could make the tanks capable of being pressurized to allow the boiling point to be artificially raised above 100 C). At some point, however, you are dealing with a potential bomb, so the Captain will order "vent steam", and the excess heat leaves the ship in a stream of superheated water vapour. This could even be through the main engine bell, providing a bit of combat thrust as well.
The downside of this scheme is you are now short however much reaction mass was vented, and of course superheated steam could cause corrosion or damage to engine parts when it is vented (if you choose to vent through the engine nozzle. if you use a dedcated port remember there will be off axis thrust being applied to the ship).