Decimal vs. Bigint memory usage

Dear List,

we are currently updating our application to use multiple database
backends, the main backend on our site will be Postgres, though.

Some of our columns exceed the limit of the 'integer' type, now we are
discussing the alternatives. Personally, I would use bigint since it
should suffice in most cases, but using decimal is under discussion,
too, because our modules would be able to precisely specify their
required column sizes.

We do not do any arithmetic on the columns, only saving and retrieval,
is decimal as fast as bigint in that case?

How does decimal store the number internally, is it a fixed size
through-out all rows and how does it compare to bigint?

Thanks in advance,
Gregor