It must be three syllables or fewer. Ideally, two
More than three is a mouthful. Our minds find creative ways to shorten most names so they flow better. Detroit is always Detroit, but Philadelphia is Philly. Royal Farms becomes RoFo. New York Times becomes The Times. Los Angeles becomes LA.
You can get ahead of this tendency and name your company the two-syllable version of whatever the long version is. So if you really want to start a business called "Federal National Mortgage Association," come up with the three syllable, memorable version inspired by it: "Fannie Mae" (derived from its initials, FNMA).
To illustrate how tedious too many syllables are, to activate Google's Home assistant thingy, you have to say "OK Google" (4 syllables). Try it for a day and you realize how tedious it is. With Amazon's Echo, you just say "Alexa" (3 syllables). See what a difference one syllable makes? Maybe it doesn't seem like much, but when you're saying it a dozen times a day, it adds up. If you absolutely must go above three syllables, they should rhyme ("seven eleven", "cee bee gee bee") to maintain memorability.
It must be intuitively spelled
Say the name out loud. Would someone know how to spell it without further explanation?
When Charles Schultz first broke into the coffee business, his venture was named "Il Giornale." The name means "journal," or "newspaper" in Italian, meant to conjure feelings of coffee being the first thing you do in the morning, like read the newspaper (like we all do, right?). But say it out loud: "visit our website at eel journahl-eh dot com." Charles would have been smart to buy iljournale.com and have it forward to ilgiornale.com. In any event, it's not a good name (not to mention, it's four syllables!). Every misspelling is an opportunity for a missed email, a missed page view, and missed business.
It does not need to describe what the company does
Apple, Intuit, Zillow, Twitter, Nest, Slack, Google, Amazon. We all know what these companies do, but not by their names. Describing what one does can take a lot of syllables. But if you can describe your business in two syllables, that could actually be a great name. Examples: SnapChat, SoundCloud, GoodNotes.
Don't use your personal name
Unless you are completely, 100% sure that your business will only ever be you (and you never should), don't lock your business potential to a person. Ever notice how law firms constantly change names based on the partners revolving in and out of the company? What a branding nightmare. Ironic, too, because the best lawyers are professionals at thinking ahead. Whatever brand equity is gained by having a name in the title is lost in confusion and a lack of memorability. All companies should strive to grow, even if they remain one person. Your business name matching your personal name limits your potential.
One last thought
These rules obviously don't guarantee business success (or failure). There are many exceptions to the rules above, - Mathematica is five syllables. - Boeing and Bechtel are not intuitively spelled. - FreeCreditReport.com describes what the company does. - Berkshire Hathaway is a combination of personal names. But these companies are succeeding despite not having ideal names. Why make it harder for yourself? If you're feeling stuck between the rules above, remember that memorability trumps all. Optimize for that.