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

Height can deviate up to 40% of the Pokémon's standard height while weight can deviate by up to 60%, often in line with the height.

There are specific odds for a Pokémon with the aura to appear in the overworld. These odds are standard but do increase as your Catch Combo increases. The standard odds are as follows