Resolution is what I want. I have scanned images with different resolutions.

That is not possible nor to my opinion very useful. A resolution is just a number, no more no less.

As an example, I have a file from my dSLR that has 5184 x 3456 pixels at a resolution of 300 ppi

The dimension then are 17,28 x 11,52 inches.

When I change the resolution to 240 ppi (without resampling it because that is a different matter that should be carefully executed) then my dimensions have changed to 21,6  x 14,4 inches but my pixel dimension still is 5184 x 3456.

And it is pixel dimension you have to search for.

You can add extra criteria so if you want files that have 2000 pixels or less then choose dimension and fill in less or equal 2000 and you get all the files smaller then that.

Might be needing an extra calculation to rule out portrait and landscape differences but you can do so with adding an extra criteria.

If you need all your files for what ever reason to be on 300 ppi then create an action in PS. Open 1 file and start a new action to record. Choose menu image / image size and change the resolution to 300 and be sure to have unchecked the resample image option (because that would change the dimensions and the file size and you don't want that in an automated process for different sizes.

Hit OK and maybe even include a Save As command (don't change the filename otherwise they overwrite each other with the same name) in a new location to leave your originals untouched and hit close and stop the recording.

Then in Bridge select the files you want to change, go to the menu tools / photoshop / batch and choose your new created action. Hit run and all the files will be set to 300 ppi.

But it is just a number that without the pixel dimensions has no meaning at all.

It is very popular for clients to ask for 300 ppi but if you send them a post stamp size at 300 ppi the can't make a nice print out of it.

Did the method described in post #5 to search for resolution not work?  All Metadata field is at very bottom of Criteria dropdown box.

Since Filter has only a limited amount of categories you would have to be creative to use Filter to get what you want.  Also it only filters the images in the current folder so this is a restriction if you want all images of a certain resolution.

Suggestion: make your own code for a keyword to represent resolution.  Filter works well on keywords.  You could use the Search to bring up the images by resolution and then keyword them.

In addition, if you have them keyworded to resolution you can then use Search if you want to look outside of the present folder. 

But think what you really want to achieve for obtaining images with a certain resolution.  As Omke and I have pointed out resolution is really a non-event as it is a variable that just dictates how many pixels per inch are printed to the screen.  If you are interested in the quality of the picture you need to concentrate on the pixel dimenstion.  As I pointed out an image at 3800 x 2600 pixels at 100 ppi is a better picutre than one at 1200 x 780 at 300 ppi.  Here is a link to add to the issue http://www.steves-digicams.com/knowledge-center/coming-to-terms-with-d pi-ppi-and-size.html

This is not intended to tell you what to do as we have no idea of your needs and knowledge of the issues. 

And of course in the Sort options on right side of toolbar once can sort by resoultion, file size, dimenstions, etc.

PS. But if we talk about images from a camera, resolution is of course totally different thing. I usually dont care about the resolution at all. I look at the number of pixels. As you and Omke pointed out.

Well, here it becomes a bit tricky because I have not scanned for a long time but basically to my opinion it is the same principle. If you scan a negative at 300 ppi it creates a much smaller file then when you do so at 2400 ppi but this still only reflects the width and height because your original (from slr) is only 24 x 36 mm and  the higher the ppi setting in the scanner the more data it will create for details.

That said, just try a scanned file and change the ppi in PS Image size to 300 ppi and be sure to deselect the resample option and the result should be still a file from the given pixels at the start. So I still think image size is the correct sorting option for you :-)