Printing Raster Images in Grand Format

If it's going to be printed in sheets use what image you have and don't worry about res. Scale it to the needed size and then make your print sections with overlap etc. Export as tif using LZW compression 150dpi max.

 

I am late to responding, guessing you moved ahead with production.

What is "the best image I can get"?
If you're starting with a jpeg your are hamstringed from the get go.
Is it a photo? Is it artwork? What file format is the original image?

Have you done any image editing...aside from trying to fix the resolution?

I would avoid "smart blur" It doesn't solve resolution issues, it just makes it look crappy a different way.

When you resize the image in photoshop...are you using the "preserve details 2.0" option in resampling...or just "automatic"?
Preserve details 2.0 is actually pretty damn good.

The trick we often use is we will overshoot our target resolution by 2 or 3 times, then step it back down.

Another important thing is when you apply a noise layer...be sure to use monochromatic noise.
Then you won't get that yellow and pink crap in your highlights.

Lastly, ALWAYS use tifs or .psd for your final output. You want as little compression as possible.
If you go through and try to combat resolution issues, then save as a jpeg...you are just adding more compression on top of compression.

The compression artifacts are what you are battling...not the resolution.

 

Get a trial of On1 or Gigapixel. While Adobe stuff has gotten better it's no match for dedicated programs.

I've been using On1 for years. If you have a solid image to start with 1000% bigger is very doable.