Frank Romano at RIT did an interesting thing. Curious about the mailing cost per square inch of post cards, he started calculating.
What he learned -- contrary to what you might expect -- was that larger cards with higher postage rates are more cost effective. A 6-1/8" x 11-1/2" card costs $.005 per square inch of print area. A 4-1/2" x 6" card, on the other hand, costs almost twice as much: $.009. (based on $.37 and $.23 rates)
Imagine, with all the 'real estate' of the larger card and incorporating Conceal & Reveal specialty printing, too, the impact you could have...
And then take it to the extreme: consider an oversized pull-tab. A 6-1/8" x 11-1/2" pull-tab post card offers a whopping 100 additional square inches of print area inside the tab, at an overall cost of just $.002 per square inch!
Printing costs for such a piece would be higher, but not necessarily by much.