Linda,
Just design your report for those measurements. In report design mode select
View/Ruler, and set the report area to those specifications.
I assume you're using the printer envelope feed, so you may have to set the report up
for landscape.
You may be referring to using the report wizard, where there may not be an "exact"
postcard size definition. Use the closest to it, and then go back into report design mode
after the wizard and make any necessary dimension changes/ field positioning changes
manually.