How do you generate a highpoly sphere procedurally with a mesh collider that fits?
If it should be a perfect sphere you shouldn't use a meshcollider. Meshcolliders are slow and can't collider with other meshcolliders. Just use a SphereCollider.
To create a sphere mesh you can just use some trigonometry (sin / cos) to calculate the vertices. To setup the UVs take a look at the UV mapping article on wikipedia.
Maybe you can go a bit more in detail what you wanna do. If you want to create a planet surface then you would need a meshcollider. After you created your mesh, just assign it also to the meshcollider. That should be enough.
If your mesh collider is smaller than your sphere you can atach the mesh collider to an empty gameObject then just scale it to the same size of the sphere and attach it.