LOD techniques

Im trying some LOD techniques. One of the techniques requires the existence of multiple 3d models for the same object. But to know how many models are required i need to know a few things.

One is to able to calculate the size of a pixel in the view plane. Lets say that field of view is 45 degress, the plane distance is 1000 meters and the screen resolution is 800x600 pixels. How do i know the size of the pixel in meters ?

Another technique requires replacing an individual object by a flat image. What variables do i have to look at to know when to replace a 3d object with a flat image representation ?

Another technique is to palce a group of objects in something like a “system”. This technique is the only one im considering for deleting and adding objects to a scene dynamically. Lets say i have a set of objetcs for a group of birds. When this group of birds is seen faraway the system contains less objects. As anyone done anything similar to this ?