Compositing 2 images

[defaultmljqib3jf3py]

Attempting to create a Milky Way image.  I have 2 images - 1) a foreground landscape 80% foreground 20% sky and 2) a sky image 90% sky and 10% foreground.  The resultant will be at least 2x the size of either of the single images.

I can do a sky replacement on the foreground, but I lose 80% of the sky replacement.  I've tried to increase the size of both the image and the canvas.   I need a workspace that is substantially larger than either of the individual images in which to composite them into a single resultant image

[Conrad_C]

What is not working about increasing the size of the image or canvas? Is it displaying an error, like a size limit? Because normally, increasing the canvas size should give you the additional room you need.

[defaultmljqib3jf3py]

Increasing either the image or canvas size produces no additional room. I expected by increasing at least the canvas size that when I pulled in the sky replacement, there would be room to accommodate the additional area of the "sky replacement image".  That seems to not be the case.

In rethinking all of this, I'm going to pull in each image separately, and then create a new image/canvas of size to be able to accommodate both - then go from there.  ---- I think, perhaps, maybe....

[Trevor.Dennis]

There are so many ways you could do that.  Conrad mentioned Canvas size (Ctrl Alt C),

You could create a new document and add both images to it.

You could use the Crop tool, and drag the handles outside the initial boundaries.

You mention the finished image being more than the combined size of the source images.  Do you intend stretching the images to fill that size?  That's going to lose some image quality.  If you havr the beta version installed, then you could fill the gaps with Generative Fill.

You mentioned Replace Sky.  One thing it does is produce outstandingly good selections of the sky/foreground interface, so I often use it just to to get that selection to make a mask for a composite. 

I am not sure what you mean when you said you are losing 801% of the sky replacement.  Do the donor images have wildly different pixel sizes?  If so, you are going to have to downsize (Free Transform) the larger one to fit if you want to avoid losing quality of the smaller image.

If you don't mind showing us the two images, it will make it a lot easier to help you.