You've received good advice so far. Generally speaking you do not want to crop too far. BUT everything is relative, and it is CERTAINLY the case that you can go further with images that were captured with good technique, good support, good lenses, etc. Most beginners overlook this consideration - which is actually a much bigger contributor than it may seem.
Also, not for your immediate use, but do be aware that software is now good enough to print at least twice as big as you've seen with generally very high quality. Depending on the image and how well it was captured, processed and printed, I've seen 4mp originals printed beautifully at 16z20. Of course, doing this requires the very best all the way through the chain, but it can be done. Your D3100 can produce a 14mp file.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!