Resampling and making a huge change up in number pf pixels does not work well there is just so much detail you can create without the image becoming very soft.
JJMack is truly giving you correct information. If the computer has to "invent" pixels that don't exist, you're going to get all sorts of blurriness and random pixels - it's never a good idea. However, sometimes, it HAS to be done (eg: I did a job for a lawyer who needed iPhone images to print at poster size for court). Since Photoshop's algorithms aren't great for that purpose, I used a 3rd party plugin. I had really workable results using On1 Resize 10.5 (I actually preferred their version 9 product, but that's not available anymore). It gave me control over sharpening and the resizing algorithm. OTOH, I must admit that some of the images wouldn't look good close up, BUT at poster size, they weren't meant to be viewed that way. You may get very good results doing much smaller enlargements.
To the moderator: Please delete this, if mentioning another company's product is unacceptable.
I am well aware of the limitations of upsampling.
I looked at On1's product, and it does a decent job, although I can't tell what algorithm it's using, and it wasn't better than Ps's "preserve details" setting. It was much softer, with less detail. But thanks for the recommendation.
And sorta true about the images close up. But the devil is literally in the details; for say a slide show on a projector the upsampling for consistent size is less noticeable than pixel peeping on a retina screen from 8" away.
Okay. You'd asked what others had done. I hope you find someone who can offer something better for you.
This underscores why you need to keep original hi-res PSD files. It's much better to start with high quality images and ExportAs a smaller JPG. The reverse will never produce good results.
If you don't have good quality originals, you may need to find or shoot some replacements.
I agree keeping hi res files and shooting replacements is nice. But unfortunately I haven't found a time machine to do that . These are historical images, some going back almost 15 years and sub-2MP, even before they were cropped. As well as old scans from small prints, which are now lost to us.
We've been trying other tricks; you can sometimes actually get a nicer upsampled print than you get from just exporting a resized digital file. Probably cuz some printer drivers are very good at sampling and they're not confined to digital pixels. We then rescanned those; can make a nice effect. But it's too time consuming. With some resizing and some work to make them look kind of retro helped. For our purposes a bit soft and even kinda out of focus works far better than any pixellation. From the context and the content one knows they're old, so people aren't expecting pixel-peepring sharpness. Especially when viewed via projector, as S_Gans noted. And BTW, thanks for the On1 suggestion, that's why I tried it. Just didn't give us appreciably better results on the trials.
Glad to see you found a workaround. With the On1, I had to play a lot with settings to get things right, but for me, overall, it did better than PS's Preserve Details option, once I found my settings for each image size I was given.
Keeping a retro feel is a good compromise. Glad you found it.