A bigger sample is usually better. The ability to detect true differences when they exist.
Power is the probability of rejecting the null hypothesis when it is false.
Power of at least .80 is considered acceptable. Prior to collecting and analyzing data the sizes of treatment effects and error variance are usually unknown. These values can be estimated from sample data or pilot studies. But, generally you should try to collect the largest sample size that you can given the resources that you have. 1 - df for treatment levels.