I assume that the 7 in long by 7.5 in wide is the page size you have set for
the report.  The maximum size of the report section will be the page size
minus the sum of the top and bottom margins (so, should be 6.5 in).

This size will be the total length of all the visible sections in your
report on each page; it will include the Report Header (prints only on the
first page), the Report Footer (prints only on the last page - which may be
the first page if the report is short), Page Header and Page Footer (print
on every page, unless you've set not to appear with Report Header/Footer in
the properties box for the report itself), and any Group Headers/Footers
which you may have.  It seems that you probably have one (or more) of these
sections taking up space, and preventing the Detail section from becoming
longer than 6 in.

HTH,

Rob