from Wiktionary, Creative Commons Attribution/Share-Alike License
- adj. (of a syllable) Having two morae, i.e. being composed of either two short vowels, or one long vowel, or a diphthong.
bi- + moraic (Wiktionary)
Well, you're probably at least as well informed as I am, but what I think they mean is that bimoraic syllables are written with two characters, so the number of characters is equal to the number of morae represented.