Size doesn't matter at all. As for temping, you won't be albe to tell you are going to ovulate by taking your temp only. You may get an ovulation dip in temp the day you ovulate but not always. The only way to tell that you have ovulated is to get a sustained thermal shift in your temps. At least three temps above your coverline temperature. Your cervical mucus is more important to tell if you are about to ovulate. It is a bit confusing I know. Do you have a library close to you? You can read about how to use temping in the book taking charge of your fertility.