We take samples to study populations because is impractical, and most of the time, imposible to study the whole propolation. For example if the population is very big or infinite. Or if you don't have easy access to the individuals of interest or economical reasons can also determine the sizes to take.
As the populations are different in size and compositions, sample sizes cannot be standardized values, the sample size always depends on the size of the population.
It is normal to consider that if the sample represents 10% or 15% of the population then it is representative.