what ratio is this?

The idea pixel size for the thing I'm trying to do is 2516x1158

But what ratio is that?  

I want to work with the raw photo then scale it to the desired pixel size at the end.

The ratio is 2:1 or 2.17271157:1 to be more specific.

2516/1158 = 2.17271157

This doesn't fit a standard aspect ratio.  The closest is 16:9 if you fudge some pixels.  The good news is you have a lot of extra pixels.  It is much easier to reduce pixel density than to add.  

You're gonna have to trim some off the top and bottom to fit a 16:9 aspect ratio.

Assuming you are talking about a standard video resolution.  If not, please disregard what I said.

Those were the 'bleed' dimensions.
The trim is 2481x1122px

Looking at still photos and trying to get them ready for something like (but not exactly) a print on demand service.

I know they adjust the photo a lot once I upload it, but I want to start with something as close as possible so they don't degrade the image quality.

R, what is the pixel measurement of the photo you want to work with?

I would scale the photo first to either 2516 px length or 1158 px height and then crop it to the desired size. Would that work? Or if the original photo is larger, crop what you want to the required measurements.

EDIT: I'm assuming they've told you to change the resolution to 300 pixels per inch (the standard for print photos.) You need to do that first if you will be resizing the image, because 2516x1158 will be a different print size at 300 DPI than it would be at say, 72 DPI. Of course, if that's information you don't need, please ignore!

Ah... so it turns out the measurements they gave me was for the entire physical thing, not the individual photo part.  UG.

So I put it on my screen, got out a ruler, and then guessed it's 4:5 ratio.  It worked out great.