Two to six words works best. Short enough to read in one glance, long enough to convey an idea.
One-word slogans are often too abstract (what does 'Connected' mean without context?). Slogans of seven words or more are usually sentences rather than slogans. They lack the rhythm that people remember.
Well-known examples almost always fall within this range. The test: can you say it in one breath without stumbling? Then you have the right length. Do you have to pause? Make it shorter.