If the gene pool of the founding generation is of high quality, population size isn't a major factor.
Most popular notions of the ills of inbreeding aren't accurate when it comes to populations founded by a strong hardy stock. Only when a significant number of poor genes are present at the start will a population run into problems later on.
Considering that they started out with such a small and technologically simple beginning, the American Indian populations did very well for themselves in the preceding 13,000 years. No doubt there's plenty of lessons to be learned for future space colonists.