[Dovecot] rule of thumb for indexing overhead

I realize it's hard to be precise about this, but does anyone have a feel or rule of thumb for a couple of aspects of indexing overhead?

1. Proportionally, how much space does it take for all 4 files? If I want to give my users a quota of 100 MB for messages, how much real space should I plan for so that I won't run out of space for indexing?

2. What's the overhead in rebuilding index files? Suppose I use an LDA other than dovecot, so at least the INBOX index is frequently getting out of date. Does it cost much (in CPU, memory, and disk IO) to rebuild the index files? (I'm using maildir.)