as I pulled out my roofling I discovered some rust that I would like to get repaired, I will attach some photos of the rust, but if anyone could give me tips or idea on what I should do.
I was lucky enough to get my hands on a free shell to cut out what ever I need, but I have no idea how much to cut etc.
if anyone could give me tips or advice that would be much appreciated, cheers in advance.

That is the smallest pic I have ever seen, and I can't tell what area of the car it is. Generally with rust repairs, to find out how big a hole you need to cut, you should hit it with a wire wheel on an angle grinder and get it back to clean shiny metal. Usually you will find more holes in the near vicinity. Then, you just need a section large enough to cover all the holes. If possible, I would let the person doing the welding cut the patches out of the donor shell. If that's not convenient, just make sure you cut the patch nice and big! It can always be trimmed later.

Area is just above the wheel arch. Is that black area a previous repair? May pay to check out whats lurking behind it? Put in one larger panel for the whole area............

It is a pretty simple repair area. It can take a bit of time to do right thought. Being inside under trim, it does not have a critical requirement to be a perfect shape. No bog will be needed. The important part is to look through the hole once the rusted area has been cut out, and you will be able to see the inside of the outside panel. If it has any rust at all, you will need to replace it if it has rusted through, or clean and treat it if it is just scabby and scaly.

Cars rust from the inside out.

strange place to rust??

I would be trying to find out why??

That would be where the expanda foam was put from factory, and that stuff causes rust very badly

The foam absorbs moisture and rusts from the inside out.

The foam was a way to stop some of the engine noise from travelling up the rear pillar cavity and into the roof cavity above the headlining. By sealing the main body interior cavity from the engine bay cavity, it suppressed the noise and some oil and or petrol smells. It also allowed the main body interior cavity to be sealed so that the flow through ventilation system could operate better. The crescent moon shaped vent is an outlet due to low pressure on the outside of the vent. This would allow the fresh air from the front vents to flow in better as they were high pressure. Commonly called flow through ventilation.

So if you remove it you make your Beetle air flow inside behave like a 60's Beetle. No big deal now, but in the 70's it was important to compete on sales with small Japanese model cars.

If you are really concerned, you can get a sealed 'pillow' of foam in a Glad zip bag or two and stuff them up inside the cavity after treating and sealing the rust. However be careful that the pillow cannot slip down and get onto the engine and cause problems