Since you know you have a weasel problem I think you need to build for it. That means no opening gap more than 1/2". That is hard to achieve. You could use a solid material like wood or metal or 1/2" hardware cloth or expanded metal, something like that. Be careful at corners and connections, those can be weak points if they don't come together correctly. I would also suggest a no-dig apron, a piece of wire mesh laid flat on the ground outside the coop/run and attached at the bottom of the wall so nothing can squeeze through. That's to stop them from just pushing under the fence/wall.
Metal fencing but not sure of what to use for run “roof.”
You could use hardware cloth or something similar. That lets rain and snow in. A solid roof, whether some type of metal or plastic panels or plywood, will keep it drier but you can get snow load in Pennsylvania so it needs to be well supported. With a solid roof I'd want it sloped so rainwater runs off to a place it does not get into the run.
Any suggestions and how to attach to dog run panels?
I assume you are talking about the roof and the top of your panels are a round tube. Depends on your roof. If it's hardware cloth many people would use zip ties. They are not bad but over the years the plastic ones may get brittle and break. Maybe use short pieces or wire to twist around the connection or weave a wire thread along the length.
If you are putting on a solid roof I'd attach strips of wood along the top of the fence to give you something to attach to.
If your top is wire mesh it will come in strips. I'd overlap those a bit and use hog rings or J-clips to hold those strips together. Or use wire, either short strips twisted or a long strip woven.
Also do dimensions of coop and run sound good?
For 16 chickens the coop size isn't bad. How are you building it? If you are buying the raw materials and building it yourself, most lumber and plywood comes in 4' and 8' dimensions in the cheapest versions. If you are using paneling you need to check. If you stick to those basic dimensions you can usually build efficiently with less cutting and waste. A 2' dimension isn't that bad, you can often use the cut-offs for nests or something else. If you carefully lay it out you can usually build an 8x12 for very little extra cost and with less cutting. If you are buying a prefab coop then all this has already been priced. It may apply to the run material.
In Pennsylvania they will be sometimes restricted to the coop only due to winter weather, snow and wind. An 8x10 would probably be sufficient but a little extra room would not hurt.
I personally would want a larger run. Granted there will be times in the winter where it is not available, but I find in general the tighter I squeeze them the more behavioral problems I have to deal with, the harder I have to work, and the less flexibility I have to deal with issues as they come up. One example of that. With that many chickens pooping in that run, the smaller it is the more you will have to manage the poop as it builds up. It is a little more expensive when you build it but it might save you a lot of work down the road. To me, my comfort and convenience are worth a lot.
Good luck!
