In contrast to other cell organelles, the size of chromosomes shows a remarkable variation depending upon the stages of cell division.
Interphase: chromosome are longest & thinnest
Prophase: there is a progressive decrease in their length accompanied with an increase in thickness
Anaphase: chromosomes are smallest.
Metaphase: Chromosomes are the most easily observed and studied during metaphase when they are very thick, quite short and well spread in the cell.
Therefore, chromosomes measurements are generally taken during mitotic metaphase.