I got this question to many times and it made me search to much as well. So I thought lets put it onhere so every one including myself can find it quick and easy.
Initial size of the Microsoft CRM 3.0 server installation
C:\inetpub\wwwroot\crm = approximately 25 MB
C:\Program Files\Microsoft CRM = >approximately 100 MB
Initial size of the Microsoft SQL Database
C:\Microsoft SQL Server\Data\databasename = 50 MB