> .... I believe there's some documentation that came with VFP describing the structure.

repo form HOME(1)+'Tools\Filespec\90frx.frx' preview

Beware! The order of records is important!

The hight of summary might be as complex as any other section of the report. Simple ones without dynamic height changing and self hiding stuff you might gather from frx.
If you need to deal with dynamics, try to dive into report listener - it might be the best place to get the height of a report section.
Anyway, I do not use report summary band at all - I wrap the whole report in a group like .F. and use the summary of this group. Gives better control.