Population sizes are mostly driven by the anticipated effect size of the intervention on an outcome. In other words, if an intervention is expected to produce a large effect, the population can be small. Conversely, if the intervention is expected to produce a small effect, the population must be large.
Other factors can also drive enrollment size, for example, cost of the trial, potential incremental revenue derived from positive results, the need for an adequate number of patients for a safety database, and the total size of the population affected with the condition under study (which drives trial size both in terms of the amount of money that will be devoted to the trial and the ease and speed at which patients can be enrolled).
