thumbnail size

You can change the thumbnail size so that the Finder can view larger thumbnails but it's Adobe's responsibility to let Photoshop render the thumbnails larger. p

However, you can use the Quicklook from the Open window in Photoshop to view the thumbnails much larger and the arrow keys on the keyboard to move from one image file to another:

Just select an image and press the Space bar. Cycle thru the files with the arrow keys.

First activate the thumbnail view in the folder you're working in. Next select the View ➙ Show View Options menu option:

Then you can make the thumbnail as large as you want up to 512 x 512:

Ok, I am a little confused:

Finder is a Mac function and means you have nothing open - your desktop/background is showing.

Photoshop "open" menu is when you are actively within the Photoshop application.

Can you post a screenshot of the window you are referring to?

You don't say which version of Photoshop you're using! I'm using Photoshop CS6, and there is no way of adjusting the size of icons in Photoshop´s "Open" dialog as you can in a Finder window. Adobe "support" saying this is a Mac issue is just them typically passing the buck.

Try this: when in Photoshop´s "Open" dialog have the view set to "column" view, and when you select a file you get a decent-sized preview on the right. Not exactly what you want, I know, but it might help! Personally I always just use a Finder window for selecting files to work on in Photoshop, but everyone has their own way of doing things! Regards, Ian.

As Photoshop is a third party app, you will need to look for preferences within that app or contact their support. If it is in Finder, you can adjust the icon size in its view options.