Not sure what your report looks like, but I assume that there are varying numbers of detail lines with a header and footer.
If I am correct, if the number of detail lines exceeds "x" the form will exceed the normal size.
I would use a record counter which resets when your group changes. Check the detail line to see if the record equals "x"+1 and do a form feed.
To do this create a Running Total Field which "counts" the any detail record field.
In the section export used a formal in the New Page before or after which checks for the the maximum record.
Hope that helps.
Dave