How is your report structured? Does your letter consist of a group header, details section, and group footer? When there is only 1 detail row in a letter, where do you want the extra space to appear? If you want the entire letter to be reoriented so that it is centered vertically on the half page, then I think you could insert additional group header and group footer sections that are conditionally suppressed based on the number of detail rows. If five is the maximum number of lines, you could add two group headers and two group footers, and conditionally suppress GH#1a with:
count({table.groupfield},table.groupfield}) > 2
Suppress GH#1b with:
count({table.groupfield},table.groupfield}) > 4
Suppress GF#1c (the last one) with:
count({table.groupfield},table.groupfield}) > 1
Suppress GF#1b with:
count({table.groupfield},table.groupfield}) > 3
-LB