estimating size of bitmap index

Hi,

I am creating a bitmap index and I have been following one of the procedure given in one book and i feel that the size of the bitmap index should be smaller than the b*tree index.

1. How to estimate the size of BITMAP index ?.

2. Is there any view which can tell that how many empty
blocks are available in the index ?.


I would really appreciate a quick reply


thanks

gtm