The Scott U.S. Specialized catalog describes the format and margins of these souvenir sheets. It would be good reading before purchasing a sheet. There are large numbers to choose from in the marketplace. Shop around and get the one that's right for you.
But a picture is often better. These were printed 4 souvenir panes of 25 to a press sheet of 100 and then cut into quarters. As sold, each pane of 25 will have 2 wide margins depending upon which quadrant you are looking at. The upper panes will have a diagonal corner notch. I have taken 4 images from the ebay listings for plate 18771 and reconstructed a press sheet to give a visual:
There is a trade-off between those wanting the pane as-issued (with two wider margins and a 50/50 chance of a corner cut) and those wanting the symmetry of a post-sale trimming (at what should be a much reduced price). Either way, one should approach a purchase with the knowledge of what one is buying.