mysql index size and duplicacy questions

Question

Hello, I have a few general questions about database Index. Answers to any question are awaited:

1. How to calculate total memory size occupied by an index. And is the size of index directly proportional to its Cardinality ?

2. Do we need to index primary key, or is it indexed by its own. And how will it effect the memory and processing speed of select,insert,update queries if primary key is indexed twice (first automatically while creating table, then manually with different name) ?

3. Can we arrange or control the seq_in_index while using combined indexes. If yes, whats the best way to do this with respect to the cardinality of individual indexes ?

Thanks a lot in advance!!