I have never received a telephone call form anyone complaining that they bought too big of a heater. Basically the heater should be sized correctly for the pool. But with bigger heaters you have the option of using your blanket less and the pool will get back up to a comfortable temperature after you have bad weather. The cost at the end of the month is more related to the efficiency of the heater "COP" So if a small heater and a big heater have the same COP then the small heater will run longer and use less power when it is running and the big heater will run less but use more power when it is running so at the end of the month the electric bill will be the same. Not an exact science as you don't want a heater so small it will never shut off. But there is more to choosing a heater than just that. You want to look at the warranty in your area, the efficiency of the heater, the reliability of the heaters, etc.
I hope that helps.