- the size of the population S and
- the probability µ to change a letter
Obviously, the greater the size of the population for a fixed µ, the smaller the number of generations it takes to find the target. On the other hand, for a fixed population size, a similar rule for µ can't be given - one probably wants to keep the parent in the next generation, so µ has to be quite small for small populations, and can be bigger for great ones.
Here, the coloured lines represent the expected values of the number of generations for different sizes of population as a function of µ. At each curve, I added the minimal expected number - and I joined these minima by a black line. So, a population size of 200 results at best in 49 generation on average - and this minimum is reached for µ ~ 0.044.
But if you want to have a fast algorithm, you'd look at the number of queries, not the number of generations, i.e., the total number of children, or how often the fitness function has to be calculated:
(Yep, I like this pic :-) Here, you can see that for getting the best results, the size of the population is surprisingly small: 20 individuals. But though it takes over 325 generations on average to reach the target, the fitness value for only ~6514 individuals has to be calculated - that is, if you chose µ ~ 0.041.
Another interesting effect: for population sizes from 20 to 200, the optimal µ doesn't change very much, it's just a little bit over 0.04.
Of course, no one does these calculations before running the program: you write your weasel and play with it. But the natural behaviour of this algorithm will lead you to population sizes from 20 to 200, and a µ of 0.04 - 0.05.