This oval router jig Project plan set has all the info you need to build the oval and ellipse cutting router jig. It includes detailed measured drawings and a materials list. Most of the jig is built from scrap wood you most likely have in your shop.
I have not fully tested what the max size ellipse/oval this jig will cut, but I have cut up to a 36″ x 48″ oval with it.
However, if you want to cut a larger oval, all you have to do is make the swing arm longer and the base wider so you have enough travel.
Some of the specialty products I used in this build
You can watch the Router Jig be built on youtube
Elliptical Cutting router jig