Shortest in syllables: Gloom (others include Seel, Muk, Jynx etc.)
Shortest in letters: Muk (other: Mew)
Longest in syllables:
- Galarian Standard Mode Darmanitan
- Zamazenta Hero of Many Battles
- Gigantamax Urshifu Single/Rapid Strike Style
If you don't include forms, then Feraligatr (arguably Nidoran♀)
Longest in letters: Gigantamx Urshifu Single Strike Style
If you don't include forms, then Crabominable comes first
Longest in words: Zacian/Zamazenta Hero of Many Battles; Gigantamax Urshifu Single Strike Style
If you don't include forms, then Mr. Mime comes first