Question

why are human beings who took so different from each other in terms of size, colour and looks said to belong to the same species?

Answer 1

species are comes under last obligatory classification or bases of classification
we can consoder tgem as a different specues which can reproduce freely with each other
under species tgere are also some intermediate classes like sub species and races
our morphological and anatomical differances are considered under them

Answer 2

Why are the human beings who look so different from each other in the term of size colour and looks said to be belong to same species?

There are different races of human being which different in external appearance but they all belongs to the same species.

Because all the races:

1. Share the same gene pool.
2. Are capable of interbreeding.
3. Contain the same no. of chromosome in their cell
4. Have common Molecular phylogeny involving DNA sequencing.