Increase image size and reduce file size

@glemasurier 

There's nothing magic about 300 ppi, it's based on a formula for offset/lithographic printing and has nothing to do with inkjet output.

Epson output has traditionally used resolutions based around the inkjet head with common input values of 240 or 360 ppi input for 720/1440 dpi output on good quality gloss or semi matte media. Some new Epson LFP's are now using 600/1200 dpi output, which is a departure from the historical Epson head resolution.

How do I upscale the height/width dimensions in PS 2025 on a current Mac without inflating the file size. I would actually like to reduce the file size.

By @glemasurier

To achieve that takes two separate stages. The first is about increasing the pixel dimensions, the second is about how to compress the file size.

First we’ll look at meeting 300 ppi at the larger size.

If you want 20 inches high at 300 ppi, then using the Image Size command in Photoshop as a calculator says if your original file (5472 x 2884 px or 15.8 megapixels) is enlarged to 38 x 20 inches, it can be:

• 144 ppi without resampling (using original pixels only), at 16 bits per channel this gives you a native TIFF file size of about 90MB. 
• 300 ppi with resampling, resulting in 11384 x 6000 px or 68.3 megapixels. The native file size of this image, at 16 bits per channel, is 391MB.

Resampling when scaling up is also called upsampling. Making up a lot of new pixels is required to let the image achieve 300 ppi at the larger dimensions in inches. Adding all those pixels means the file size can only go up, if the file format and compression are not changed (I’ll get to that later).

When you upsample, you have choices for how to make Photoshop invent all those new pixels. You have several options here:

• Test to decide which Resample option you want to use in the Image Size dialog box in Photoshop, because each enhances details in different ways.
• Consider instead using newer enlargement features based on machine learning/AI such as Super Resolution and Super Zoom, which are in different parts of Photoshop/Camera Raw. These can potentially invent more believeable detail than the Resample options. 
• If you don’t want any new pixels to be invented at all, stick to the original pixels only without resampling. This will keep the image at 144 ppi when enlarged to 20 inches on the long side. You can still use professional sharpening techniques to enhance apparent detail. Is 144 ppi acceptable? It might be, if you take the expected viewing distance into account. 300 ppi is for close viewing, but the further away a print will typically be viewed, the lower the ppi can be. If the print will be hung so that most people will be viewing it about twice as far away as in their hands, then 144 ppi might be OK, especially if properly sharpened.

Next is the second part, where we see how far down we can get the file size.

Your original is probably 16 bits per channel, because that seems to be what makes 5472 x 2884 px a 90MB file. If it was 8 bits per channel, it would be half that file size. 16 bits per channel is appropriate for originals, but for printing you won’t see any difference if that’s an 8bpc file. But, TIFF will probably not be able to get the file size as far down as you like.

Many printing companies also accept JPEG, and if your printer has a big problem with a 90MB file (which is a little strange today, 90MB is not unusually large for a TIFF or Photoshop document), see if they’ll take a JPEG which can compress further by trying to throw out image info we might not notice. In Photoshop, if you preview the file size while saving a JPEG using the File > Save a Copy command, the 11384 x 6000 px version of your file (the one that’s 391MB as a 16bpc uncompressed TIFF) would be around 50MB at JPEG compression level 10, or 42MB at compression level 8, as shown in the picture below. I wouldn’t lower the compression further than 8 because it might start to visibly degrade enough to be seen in a print.

The 5472 x 2884 px version of your file (the one that’s 90MB as a 16bpc uncompressed TIFF) would be around 11.6MB at JPEG compression level 10, or 9.7MB at compression level 8. The file size you get may vary because it’s affected by the specific content in each image.

Note: The file size estimate shown in the picture below appears if the Preview box is selected. When it is, the document window previews the effect of that level of compression on the image quality as well as showing a file size estimate.

So it all comes down to two choices:

1. Decide whether upsampling is even necessary; if 144 ppi is enough at the intended viewing distance then you won’t need to upsample, just scale the original pixels to the new dimensions in inches. 

2. Decide whether you want to hand off a JPEG version; if the printer will take that then decide where to make the tradeoff between compression quality vs. file size.

What is the printer's problem with a 90 MB file? A professional printer should have no issue with any size file. If it's a matter of electronic transfer, you can always mail a thumb drive with the file.

I have work with a local metal printer who actually prefers the upsizing function of Photoshop over a larger file with Enhanced Resolution. Results are excellent.

My (amateurish) answer might be to save as JPG, high quality. 

A 5472 x 2884px image will print at 144ppi which may be sufficient in a large print  (37 x 20in). Upscaling (re-sampling) would be possible if JPG compression reduced the file size to a limit you require, but upscaling will not increase the resolution of detail.