Q:

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

This is the view I get when I want to upload a file to OS X' browser. I like the thumbnail view, but as you can see, they are too small. 
I don't mind showing fewer pictures – I just want a bigger thumbnail view.
How can I change that?

A:

This answer applied to whatever version of OS X I was using in 2014.
Run
/usr/libexec/PlistBuddy -c 'set FK_StandardViewSettings:IconViewSettings:iconSize 48' ~/Library/Preferences/com.apple.finder.plist
where 48 is the new icon size, and restart to apply the changes. Just logging out and back in does not seem to apply the changes.
The FK_StandardViewSettings settings are used by FinderKit and file dialogs. They are not changed when you press the "Use as Defaults" button from Finder's view options.
If you run the command on a new installation and there is an error like Set: Entry, "FK_StandardViewSettings:IconViewSettings:iconSize", Does Not Exist, open a file dialog once in some application first.

