The longest name in letters is brambleghast at 12. In words, type:null. What about syllables? I don't know the shortest names so also shortest names in syllables and letters. English only, earlier release breaks ties.
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