Is the site usage summary customizable to show the top 20 pages for example instead of just the top five pages?
If yes, how?
The short answer is no. However, I'll tell you where the top 5 value 'appears' to come from. I say 'appears' because I haven't tried modifying this to see if it makes a difference, and I wouldn't recommend doing so either.
So the Site Collection Usage summary report is located in
C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\LAYOUTS\SpUsageSite.aspx
On that page is a control of type SiteTopPagesReportControl, which in turn is in the Microsoft.SharePoint.Portal assembly. This control displays the 'top pages'
Using .NET reflector to see the code for this control, you can see that the data comes from a stored procedure called "proc_ANL_SiteTopPages"
Looking at the SharedServices database, that stored procedure contains the following.
SELECT TOP 5 h.FullUrl, h.DocName, CAST(SUM(h.HitCount) AS FLOAT)/@days AS AvgDailyHits....
Hope that helps! I'm pretty sure I don't have to tell you that you shouldn't modify this stored procedure.