No, keeping her intact will not make her grow larger. She will grow to whatever size is predetermined by her genetics (and short of conditions such as severe malnutrition, for example, there is nothing you can do to influence that one way or the other).
Having siad that, there is a small amount of evidence to suggest that animals altered early may grow larger than their intact counterparts. However, the general veterinary consensus is that it makes no practicable difference either way (and the lack of giraffe-like dogs in a world where most are altered by 6 months of age would tend to support that conclusion).
In short, the age at which your bitch is altered is not going to make the slightest difference to her adult size. It can, however, have some impact on long term health risks. A short veterinary article about that here:
http://www.peteducation.com/article.cfm?cls=2&cat=1625&articleid=926