Sadly, Monster (and, you probably already know this) the damage has already been done perhaps in this particular relationship. --There (may) be some cause to be hopeful though, if certain elements can fall into place.
First, if there's any chance to do it....try now to "build" the relationship around other "qualities" about yourself and her. Your "size" should not be further emphasized AT ALL right now--IF you can possibly avoid it.
Likely, she's going to be reminded though, of that "size factor" from this point forward--and with considerable concern, too. It may very well be she needs to be educated that size CAN be accomodated, given time and patience. She needs to desire you however for other (more important) reasons first and foremost to allow the size factor to be dealt with at a more appropriate time. If you do have occasion to get back into discussions about your penis size (as in additional conversations which SHE initiates and NOT you!) you might (try to) tell her the reason you mention it at all to begin with was to avoid shocking her. Let her know you are willing to take things slowly and be patient with her...that you don't intend to inflict pain and otherwise ruin an otherwise beautiful part of your relationship.
You may have some chance of continuing this relationship. Or, you may not. Do you even want this woman in your life? Are you serious about her?
Whichever's the case, yours seems to be a most valuable lesson learned here for others of us too.
Breaking the news about our "size" to a woman needs to be carefully calculated, timed appropriately when the information won't dominate (or negatively influence) the relationship, and discretion must rule in wording the news--trying to ascertain her possible reactions to that news with YOUR correct responses all ready to go so as to calm her fears. In other words, letting a woman know about a having a large penis should not spell out to her that it's going to be a weapon of mass destruction. Instead, the message the woman receives should be that the man behind that size will nonetheless be a considerate, thoughtful lover who will put the woman's feelings and needs first. There has to be a willingness from BOTH parties to make this work.