In the genetic drift analogy of a drunk man on a train platform, the width of the platform is analogous to what

The answer is the population size. The chance has a role in determining whether a given individual survives and reproduces and the alleles in the offspring are a sample of those in the parents. it is the change in the frequency of an existing gene variant (allele) in a population due to random sampling of organisms. Genetic Drift or another term for allelic drift or the Sewall Wright effect.