So sorry she's not big enough. As a breeder I agree I wouldn't take the chance on one that size, I'd have her spayed.
Read this thread to see both sides of spaying your female before you make a decision. I know your female is older but the risks are the same...
http://www.yorkietalk.com/forums/showthread.php?t=29229
Don't stop there!..Ask experts, talk to other pet people on other forums, read on the net. I have done all that myself and found that there are risks in whatever we decide.
Please! Just get all of your facts together and make a decison you and your yorkie can live with.
Good luck!