Apple's current guidance distinguishes version 27 and later, which stores shared-album photos in their original formats at full resolution, from version 26 and earlier, which reduces photo resolution. A blanket “shared albums always shrink photos” is no longer an adequate answer.
Check the file you actually received
Imagine wanting a print of a family photograph shared months ago. Start with the system versions involved and the properties of the saved image. Do not infer its resolution from how sharp it looks in a small on-screen preview.
Ask the person who holds the original to compare pixel dimensions and file type with your saved copy. If the origin or upload history is unclear, requesting a separate original is more dependable than guessing what an update changed for an older item.
Save before relying on it
Open the shared album, choose the image and use the available save or import command. Then find it in your own library and inspect the actual file. Merely seeing an image in someone else's shared collection is not the same practical checkpoint as having the copy you need.
Keep the original when it matters. Do not delete a personal-library copy because a shared preview looks fine. For printing or editing, verify the copy against the intended task; full pixel dimensions alone do not prove an unedited history or the presence of every piece of original metadata.