caching indexes and pages?

Hi I am developing a database and have a couple of questions I havent
found an answer to yet.

1) how do I find the size of an index, i.e. the size on disk?

2) I have a query that is taking a long time to complete because the
table is about 120GB large. Its only returning 2000 rows, so in
principle it should be fast. But because the data is spread across the
table, I am assuming it needs to do a lot of disk access to fetch the
appropriate pages. Since the amount of data is so large I am also
assuming that whenever I do a query all memory caches have to be
replaced to make room for the new pages. What I am wondering is which
config settings can I use to increase the amount of memory postgres
keeps to cache pages and indexes?

I tried work_mem and maintenance_work_mem but it does not seem to make
much difference yet. Admittedly I had set it to 100M and 80M, so after
reading a little bit more I have found that I could easily set it to
several GBs. But I am not sure those are the correct config parameters
to use for this. I havent found any others that are relevant so far.

regards

thomas