You can use NavigationInfo node and its visibilityLimit and avatarSize fields to adjust how near and far objects are rendered. Generally the smaller avatarSize, the nearer object you see (are not clipped). The bigger the visibilityLimit, the farther objects you see. The bigger the ratio visibilityLimit/avatarSize, the less detail you get rendered. Finding the correct values for these two is usually a trial and error exercise, and in your case I'd start with adjusting visibilityLimit first.
You can place the NavigationInfo node either at the root of your virtual world or make it a child of the same parent as your Viewpoint, depending on whether you want to act it globally or locally.