Well.............Micah and Asher are 5 letter, two syllable names. Not that the other name has to be a 5 letter, two syllable name, but maybe not as long as Zephaniah. Here are some other names you might consider:
Continuing on the five letter, two syllable Biblical theme, how about Silas? Simon? Moses? Caleb? Ethan? Elias is nice but it's three syllables. I'm intrigued by Zephaniah but the spelling and pronunciation problems are sure to be as weighty as the name.
I love Zephaniah... but for syllable's sake, I'd say Silas. It's unusual and fits with Micah and Asher. Asher is a great choice with ash names being so popular. Micah is great, too, with the -ah ending being a little unusual for boys' names.