How do you reduce the size of a photo while keeping all the layers?

https://forums.adobe.com/people/Emah+Duck  wrote Photoshop 2015, PC, Windows 10.  I am creating a montage 16"x20", to be printed.  It has many layers which I want to keep as layers and the size is now 2GB, which is unworkable.  How can I keep the layers while reducing the size?  File>Export is grayed out.  ALSO: how small can each layer/photo be and still show up after it's printed?

What's different in Elements?

Since you are working with layers, that means you are already in 8-bits mode (Elements does not support 16-bits for layers).

- First, let's start with the final output. 16" x 20" means that you'll look at the poster from a distance, and 200 ppi is more than enough for fine details (3200 x 4000 pixels). And you don't resize the canvas, you resize the whole image (menu Image >> Resize >> Image size.)

- Your project seems to include more than a hundred layers, which take a lot of space, especially if the source of those portraits is made from full size photos. If you want to keep all layers (for moving, rotating or slightly resizing) you absolutely must simplify those 'smart layers' (you highlight them in the layers palette, right click and choose simplify). The change in kilobytes size is dramatic! You can select (highlight) many layers and simplify in a single click.

- For text layers, it's about the same: simplify if you have many text layers unless you want the ability to still edit the text.

- Another suggestion on the previous answers is still valid: use the tiff format with a compression option.

- Avoid textured or grainy backgrounds...

Tell us if your project becomes manageable