On Windows, pressing 'Print Screen' or 'ALT-Print Screen' will capture the entire display or the active window respectively. Pasting that into a program that saves a jpeg can then allow you to post the image. I don't know how to do it on a Mac.
Regardless, go ahead and post what you're seeing in the Layout tab of LR and the page setup information from the Printer dialog. This should help diagnose the problem.
In LR when you select the PRINT module, there are a set of "Lightroom-Templates" on the left of the screen. If you don't see those, make sure you have the left panel visible, that the Templates Browser is open and the "Lightroom Templates" is open.
Select one of the templates that describes what you want (e.g. 1 8x10, 2 7x5).
If you select one that is "2" photos, make sure you have at least 2 selected when you open the PRINT module.
You should see what you selected (1 8x10 -or- 2 7x5), and what you see is what should be printed.
Not to resurrected a long ago thread, but was having the same problem with my newly arrived Epson 3880 printing only 5x7 out of LR4. Many thanks to Nikonians who chimed in to the OP as these replies solved my problem too (wrong LR4 template selected). I continue to be thankful for the willingness of folks to share information on this forum. Tom