Block heater sizing.
 

I was doing some light diesel engine reading:
http://www.man-diesel.com/files/news...5.00pr_low.pdf
(Ship engines)

Found that Man diesel recomends engine pre-heating input be roughly equal to 1% of engine output and to add more if you plan to operate after shutdown in cold climates.
Now they also recomend preheating the coolant to a bare bones minimum of 20'C but our on road vehicle engines aren't that sensitive to deltaT changes, so don't worry about it.
No wonder I always thought a single 600w heater was undersized on my 6.5L and explains why I am so pleased with my two 600w heaters and wish I had went with 1kw and 600w heaters.

So a single 600w block heater on a small 4 cylinder is not unreasonably oversized.

I recomend putting oversized block heaters on to save electrical power and fuel. Use a 600w heater on a small 4 cylinder for an hour or so before driving as opposed to leaving a 200w to 400w heater plugged in all night or using an ineffective 50w or 100w heater and getting almost no result after leaving it plugged in all night.

Actually, the only block/coolant heater available for my 1.6 TD is 550W, and I use it in tandem with a 125W oilpan heating pad. Using it for 2-3 hours raises the engine temperature by over 30°C.

I can see the advantages of larger heaters (shorter warmup time), but be careful or you might melt something (my heating element fits into a plastic housing on the coolant line).

EDIT: A better alternative to more powerful heaters would be better engine insulation. Not only will it improve warmup time and temperatures, but it will also keep the engine warmer after shutdown, which is good for EOCing and for running short errands.

My block heaters go inside the engine block, taking the place of freeze plugs.
Also my trucks coolant bypass heats/cools the oil.