You will get a wide variety of opinions on this subject. There are more things to consider than just age. Because of the small size of your female, I personally feel it would be best to wait until she is a little older but it is best to discuss this with your vet. My recommendation would be to have her spay between her 1st and 2nd heat. Most females come in heat when they are between 7 months and 1 year of age. Of course if you have a intact male in the house that could create problems so you would have to consider that fact also.
Here is a very interesting article that explains the pros and cons of early spaying or neutering.
http://www.naiaonline.org/pdfs/LongT...uterInDogs.pdf
Of course there are risks no matter what age you have your girl spayed but I strongly feel it is safer if they are a bit older. Best of luck to you.