As you journey the Kanto Region, you will notices a number of Wild Pokemon that have auras around them; these auras indicate the size of a Pokemon. If a Wild Pokemon has no aura around them, they will be the standard size of that Pokemon Species; if a Wild Pokemon has a Blue aura, they will be smaller than normal; and if they have a Red aura around them, a Wild Pokemon will be larger than usual. Pokemon can either be Extra Small, Small, Standard, Large or Extra Large, and they will be tagged as such when you enter an encounter with a Wild Pokemon.
Though Pokemon Sizes have no effect whatsoever on how strong a Pokemon can be, players will receive an extra Experience boost by capturing Wild Pokemon that are either smaller or larger than usual. If a Wild Pokemon is Small or Large, you will earn x1.5 experience when capturing said Pokemon. In that same sense, should a Wild Pokemon be Extra Small or Extra Large, you will earn x4 experience when catching said Pokemon. Players can even boost the odds of finding both smaller and larger Pokemon Sizes by completing a Catch Combo.