Restoration vintage posters

This is all about pixels. You need to have enough of them in the original file.

So the question is, how many pixels w x h is your original? That puts a limit to how big you can print it.

Also, "zoom in" is a relative term. What you need to do to properly assess image quality, is to view at 100%. That's a significant number because it means each image pixel is represented by exactly one screen pixel. That's what 100% means, it has nothing to do with size.

In other words, at 100% you see the file accurately represented on screen, pixel for pixel. There is no scaling.

Sharpening is a big subject. There really are no strict rules - if it looks right, it is right. Generally, you want to avoid very obvious halos. I personally prefer the ACR filter for sharpening, precisely because you can control halos very effectively there, but opinions vary. Again, view at 100%.

One more thing: The jpeg file format is not good for this. Jpeg compression will introduce artifacts. If you can get an original in an uncompressed format like e.g. TIFF, that's vastly preferable.