At the time of writing this answer(May 2018), It looks like USCIS has only 18 characters (including spaces if any) for the GIVEN NAME field. Your name will be printed as LASTNAME, GIVEN NAME. If your GIVEN NAME(which is usually your name + middle name) is longer than 18 characters, than it will skip any characters trailing the 18th character. In most of the cases you DO NOT NEED TO WORRY ABOUT THIS. Make sure whatever is printed is correct except missing characters.
REDDY(5 characters), VENKATAN RAMAKRISHNAN(21 characters) will be printed as
REDDY(5 chars), VENKATAN RAMAKRISH(18 chars).
Hope this helps !