Appropriate coop size

[3KillerBs]

Not really, weve only had 2 really bad snows in 15 years and no because here the price of timber has skyrocketed and it would cost me roughly twice as much if not more to build from scratch than it would to buy, plus im a 5ft 2 single mum with only limited skill on building things, small things i can manage, but something like a shed would be practically impossible for me to do alone and i dont get any help from anyone

So with little chance of weather cutting off access to the run you can be more confident about the minimums then.

Also the fact that you have ex-batts means that they're bred to tolerate closer quarters than the breeds who are selected for ranging.

Here are a few links to shed conversion coops of different types and sizes to help inspire you.

https://www.backyardchickens.com/articles/the-mulligan.74743/
https://www.backyardchickens.com/articles/aarts-coop-page.65912/
https://www.backyardchickens.com/articles/the-scoop-on-a-rubbermaid-big-max-coop.76444/
https://www.backyardchickens.com/articles/toy-shed-conversion.64879/