Chinese seals and writing

Mar 2015
1) How big are typically Chinese seals? The only high profile historic seal whose physical size I´ve found is the seal of Na: 23x23 mm square. What is the size of seals of Chinese emperors, county magistrates and other such officials?
2) Who and what are the seals identified with? When a county magistrate is transferred to another post, as was the custom to do each few years, what was to happen to the seal? Turned over to the next magistrate of the same county? Taken along by the magistrate to his new post? Discarded/made unusable, and a new seal cut for each new office/incumbent combinations?
3) What did Chinese emperors write? Since Warring States, emperors usually could write, with some exceptions - Ming Tianqi emperor was a good carpenter but illiterate, and of the Mongol Yuan emperors, it is said only one could write Chinese. But what precisely did they have to write? Emperors, and actually all mandarins down to county magistrates, were employing clerks and could rely on clerks to handwrite fair copies. Authentication of those fair copies was done by seal. Then what was handwritten by mandarins? By emperors? What is the oldest extant autogram of a Chinese emperor?