Using the powers of LM 3.00+ this is indeed possible. Simply click the
button and use the 'Horizontal Level Mode' dropdown to adjust the size of the screens you have in your level. You have plenty of options to choose from.
Choosing between hex numbers 00 and 06 are recommended for horizontal levels.
UNLESS of course you meant actually zooming out the camera within the emulator which I'm not sure is possible. That may require some level of mode 7.