You can change the code in the page.xaml.cs file. I'm not sure if there are other ways of setting it but this is how I set it:
In the example below, the zoom level is set between 0.45 and 128
private void Zoom(double newzoom, Point p)
{
if (newzoom < 0.45)
{
newzoom = 0.45;
}
if (newzoom > 128)
{
newzoom = 128;
}
msi.ZoomAboutLogicalPoint(newzoom / zoom, p.X, p.Y);
zoom = newzoom;
}