variable length - user defined types/storage place

Hi all,

in a large table (millions of rows) I am using
a variable-length user
defined type
to store relatively short field entries, i.e. the
length of certain
fields could be restricted to 2^16 or even
2^8 characters.

Now I wonder whether it would be possible
to save storage place, by
using a smaller length field, i.e. 2 Byte or
even better 1 Byte per entry instead of 4
Byte.

Is there a possibility to customize the size of
the length field or does
Postgresql already internally optimize this?

Thanks in advance,
Martin.