Here is a list of the possible sizes for every Pokemon in Legends: Arceus. It looks like Alolan Vulpix and Alolan Ninetales are missing, but as they have the exact same height and weight as their Kantonian counterparts, the sizes are likely varied in the same way as well.
In terms of size multipliers:
Legendary Pokémon do not have size variation in the game and come as their standard size.
Alpha Pokémon sizes vary in difference from the original Pokémon. Some changes are of only as low as 20% while others are an increase of over 4 times the usual size.
Thus, it doesn't look like there's any one formula to determine what a Pokemon's size variations are. They appear to be treated on a case-by-case basis instead.