Once you reach Viridian Forest, you'll start to notice Pokémon with varying auras around them in the wild. This indicates their size. Size doesn't factor too much into the mechanics but does provide a special bonus when capturing the Pokémon. The more the size deviates from the norm, the bigger the boost. The Pokémon models do not scale in battle to showcase the size.
The Red Aura indicates that the Pokémon is larger than normal while the blue aura indicates it is smaller. In addition to this, it registers your biggest Pokémon in the Pokédex. There are two different tags for each size as well. Pokémon that deviate more are tagged with XL or XS rather than the standard L or S to indicate if they're large or small. This only pertains to the height as the weight has no indicator