On most flat top steel strings, the size of the sound hole is proportional to the size of the guitar. Dread's, OM's and jumbo's typically will have a sound hole from about 3.8 to 4 inches. Parlors and O size tend to be around 3.25 to 3.7 inches. Classicals tend to be around 3.3 to 3.5 inches. Changing the size of the sound hole can change the sound of the guitar by changing the Helmholtz frequency of the box. Everything else being equal, as Martinedwards said, a larger sound hole favors trebles and a smaller sound hole favors bass. When I'm designing a guitar with a soundport I adjust the size of the sound hole to compensate for the size of the soundport.
Regards,
Ed
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