Genetic Algorithm, large population vs small one

Genetic Algorithm, large population vs small one

Im wondering if there is a general rule of thumb for population sizing.
Ive read in a book that 2x the chromosome length is a good starting point.
Am i correct in assuming then that if i had an equation with 5 variables,
i should have a population of 10?
Im also wondering if the following is correct:
Larger Population Size.
Pros: Larger diversity so more likely to pick up on traits which return a
good fitness.
Cons: Requires longer to process.
vs
Smaller Population Size.
Pros: Larger number of generations experienced per unit time.
Cons: Mutation will have to be more prominent in order to compensate for
smaller population??