The model matrix mostly describes the position, orientation and scale applied to a model or entity, not necessarily the dimensions of the geometry itself. What kind of entity is it? I think there's different ways to do this depending on what kind of geometry type it is, or if it's a billboard etc.
Although, taking another look, it looks like there does exist a private function DataSourceDisplay.getBoundingSphere
that can compute this for any given entity. This is what the viewer uses to figure out how to fly to/zoom to an entity. You can look at the code for it to see how it does it, or try to call it directly.
Let me know if you figure out a good way to do this!