Size Variation

Since Pokémon GO, Pokémon have been able to be different sizes, with Pokémon Let's Go, Pikachu & Eevee being the first main series titles to display this variation, and Pokémon Legends: Arceus the first to have Pokémon size variations be visible between the Pokémon.

Pokémon Scarlet & Violet continue with this by showing the size variations on the overworld and in battle once again, although it doesn't list the Pokémon's size in the Summary screen. A Pokémon's size cannot be changed and is determined when it is caught or hatched. It cannot be controlled in any way besides from the use of Meal Powers

Level	Teensy Power Range	Humungo Power Range
Level 1	0-127	128-255
Level 2	0-95	160-255
Level 3	0-60	192-255

Pokémon that are wild Tera Pokémon are automatically based between 200 and 255 in terms of size.

Checking Size

There is a way to check a Pokémon's size, however. In Mesagoza, by a PokéBall flowerbed in the west, you will find a person who can check your Pokémon's size for you. If your Pokémon are the biggest they can be or smallest they can be, they will be given a special mark

Group	Size Value	Saying
XXXL	255	Now THIS is a BIG Pokémon! Wow, you almost never see one this huge!
XXL	231-254	This is a serious big'un! Feels comforting to have such a bulky, reliable buddy around, doesn't it?
XL	196-230	This one seems a good sight bigger than average! It's quite a specimen—take good care of it!
L	156-195	This one seems a little bit bigger than average!
M	100-155	Look at that: a perfectly average-sized specimen! It's the gold standard for normality. Amazing!
S	60-99	This one seems a little bit smaller than average!
XS	25-29	This one seems a good sight smaller than average! It's quite a specimen—take good care of it!
XXS	1-24	This is truly a little one! A pint-sized powerhouse of a ____. What a charmer!
XXXS	0	Now THIS is a tiny Pokémon! Wow, you almost never see one this little!