Eevee weighs 6.5 kg (14.3 lb), according to its official Pokédex entry, and stands 0.3 m (1 ft) tall. The Partner Eevee from Let's Go, Eevee! weighs the same. Only Pokémon Legends: Arceus lets individual Eevee vary, from 4.16 kg for a small one up to 21.06 kg for an Alpha.
The figure is the Pokédex weight for Eevee: 6.5 kg, or 14.3 lb, at a height of 0.3 m (1 ft 0 in). Sword and Shield, Scarlet and Violet and Sun and Moon all list the same numbers. The Partner Eevee you start with in Pokémon: Let's Go, Eevee! has different stats but the same height and weight.
Pokémon Legends: Arceus is the one game where individual Eevee differ in size. There the standard is still 6.5 kg, but a small one weighs 4.16 kg, a large one 9.36 kg, and an Alpha Eevee, which stands 0.81 m tall, 21.06 kg.
Eight evolutions, all much heavier
Eevee can evolve into eight different Pokémon, more than any other species, and every one of them weighs between 3.6 and 4.5 times as much as Eevee. From heaviest to lightest: Vaporeon 29 kg, Umbreon 27 kg, Espeon 26.5 kg, Glaceon 25.9 kg, Leafeon 25.5 kg, Flareon 25 kg, Jolteon 24.5 kg and Sylveon 23.5 kg.
They are also all much taller, between 0.8 m (Jolteon and Glaceon) and 1 m (Vaporeon, Umbreon, Leafeon and Sylveon). The Pokédex explains the variety with Eevee's unstable genetic makeup: Violet says its genetic code is irregular and that it may mutate when exposed to radiation from element stones, its name for the stones that make it evolve.
Gigantamax Eevee
In Sword and Shield, Eevee has a Gigantamax form listed at 18 m (59 ft 1 in) or more. Its weight is not given: the games show ????.? kg. PokéAPI fills in 1,000 kg, but that is the placeholder it uses for every Gigantamax Pokémon, not an official number. The Shield entry says the giant Eevee tries to play with anyone around and ends up crushing them with its immense body.
What its weight does in battle
Low Kick and Grass Knot get stronger the heavier the target is. Anything under 10 kg takes the weakest version, with a power of 20, so both moves are a poor choice against Eevee. Its evolutions sit right on a boundary: Sylveon, Jolteon and Flareon (25 kg or less) take 40 power, while the other five take 60. Even an Alpha Eevee, at 21.06 kg, only takes 40.
Heavy Slam and Heat Crash work the other way: the more the user outweighs the target, the stronger the hit. Against a 6.5 kg Eevee, any user heavier than 32.5 kg (five times its weight) gets the maximum power of 120.