What size wheel cylinder brake shoes and brake drum do I need to purchase?

That will depend on the sub model, WRX or WRX STI or the regular Impreza? Here are the part numbers for the regular Impreza.

Drum 26740FG000 $137.30
Wheel Cylinder 26705FG000 $55.28
Shoes 26694FG000 $99.95

Here is how the job is done in the images below. Also, this generic guide can help as well:

https://www.2carpros.com/articles/how-to-replace-rear-brake-shoes-and-drums

and

https://www.2carpros.com/articles/how-to-replace-a-wheel-cylinder

Check out the images (below). Let us know how it goes.

Okay, then these part numbers should be correct. Let me know if you need anything else to get the repair done.

Sure, here is it on Amazon for $16.00:

https://amzn.to/3SPqx3r

Happy motoring.