Are some golf drivers longer then others?

Yes, depending on what number they are.

I don't think that drivers of the same number are usually longer than each other, though.

There is a standard length but not all manufacturers follow the standard. So the same number of driver from two different makers may well be of different lengths. And it doesn't end there: Steel shafts are generally 0.5 inches shorther than graphite. Custom shafts can vary by up to 3 inches. Ladies and junior clubs are also different lengths.