Stress lengths in words are not about the length of each syllable in letters, but how long you stress the sound in speech or in your mind.
Ends of words often get stressed more.
Although “and” has three letters and “so” has two, “so” is stressed longer.
In Shakespeare’s sonnet,
Short Long Short Long Short Long Short Long Short Long
Shall I com pare thee to a sum mer’s day?
“Shall” is a longer word than “I”, but “I” is stressed for longer.
Try saying a long “Shall” and a short “I” and you will hear how wrong it sounds.
Derivatives of the same word can have different stress patterns. For instance, “equal” and “equality”: