How can I increase the size of thumbnails when opening files?

Have it your way. First this is a user to user forum. No one here works for Apple. I don't know any developers at Apple to tell. If you think it's "simple" just add a slider to the open save dialogs you should try it.

I explained a very simple method to attach images to Gmails in a browser using thumbnails you can see. That's all I can do. Good luck.

I'm using Mac with OS Sierra, and I have exactly the same problem (micro thumbnails) when trying to insert a photo into Twitter.

I think I understand why but not how to solve it.

The problem seems to be that when I select "insert photo" on Twitter (or email, etc), I'm directed to a Picture of Photo Preview mode - and micro thumbnails are the only option.

I have and can adjusted the size of thumbnails in the Photo app, and the Finder/Photos and Finder/Pictures directories, but this has no effect on the Preview mode when trying to insert a picture in a third party app like Twitter.

I've opened the Preview app from Launchpad but can't vary the size of thumbnails there.

The problem seems to be that when I select "insert photo" on Twitter (or email, etc), I'm directed to a Picture of Photo Preview mode - and micro thumbnails are the only option.

Just drag the photo into the app instead of using the "insert" or "attach" command. You can drag from the Finder or Photos. If the app or website doesn't have a drop box, you can just drag the image from the Finder into the open file dialog. Photos doesn't allow drag and drop into an open file dialog, so it may not work in other instances. It does work with Mail.

The preferred option would be to insert photos FROM the Photo app. But for some reason it the insert button defaults to the "Preview" app.

That said, drag and drop from the photo app works for Mail (that may have something to do with it being an Apple app).

With some tweaking I MAY be able get it to work with Facebook. My first attempt ended up in a HUGE photo taking over FB (which I had to shut down to get out of the app). But at least there was a photo, so there's hope. I may be able to tweak the image in the Photo app before dragging and dropping to make it work. But it's a bit "handraulic" and 19th Century!

Drag and drop doesn't work at all with Twitter - all I get is a local file location showing in the draft tweet.

With some tweaking I MAY be able get it to work with Facebook.

Not sure what tweaking is necessary. There are extensions for posting to Facebook and Twitter. If they do not show up in the Share menu in Photos, enable them in the Extensions System Prefs.

I'm not sure you understand the issue - the problem is not sharing FROM Photos. It is selecting a photo from WITHIN a social media app.

In particular, if you draft a Twitter response (and share is not an option) then select the insert media icon, you are taken to what looks like Finder/Pictures but with limited control other the View than Insert or Cancel. You cannot change the size of the thumbnails which are too small to identify in a file of thousands.

I know how to change the size of thumbnails in Finder/Pictures - but it doesn't work when coming in from another app. As I said previously, it seems to be some sort of Preview view. I also know that I can enlarge thumbnails by selection, but that doesn't help when you're scanning thousands of thumbnails.

What seems to be required is that (1) the default setting needs to change to take you to Photos - and not the Finder/Pictures Preview - but I doubt that's possible, or (2) more control over thumbnail size needs to be given over the Preview view when accessed from an app like Twitter.

I sit with the exact same issue. I work with loads of photos every day and my eye sight are not that great I struggle to see the small icons that load when wanting to upload photos or files to email, or FB or any website for that matter.

Tried everything but have not found a solution.

Go to the menu bar and select View -> Show Status Bar

Right-click in the folder and select "Show View Options." In the menu box that pops up, you can change the icon size of the thumbnails in that folder.

How does visually impaired people use an Apple mac? Amazing Apple has not sorted it yet.

PikiPiki - you got it in one and your solution worked a treat!! Thanks for your help.

Yes, I agree. Just incredible. Apple can cause so much pain. Probably some patent that they are not willing to pay 1 Cent for.