Question on Byte Sizes

Hello,

* I need information on the size of pg ARRAY[]'s :

I did not find any info in the Docs on this.
How many bytes does an array take on disk ?

Is there a difference between an array of fixed size elements like
integers, and an array of variable length elements like text ? is there a
pointer table ? Or are the elements packed together ?

Is there any advantage in using a smallint[] over an integer[] regarding
size ?

Does a smallint[] with 2 elements really take 12 bytes ?

* On Alignment :

The docs say fields are aligned on 4-bytes boundaries.
Does this mean that several consecutive smallint fields will take 4 bytes
each ?
What about seleral consecutive "char" fields ? 4 bytes each too ?

I ask this because I'll have a lot of columns with small values to store
in a table, and
would like it to be small and to fit in the cache.

Thanks for any info.