How can I see large thumbnails of the files I want to attach to an e-mail?

When I want to attach a file (an image) with Evolution, it's very hard to pick the good file because thumbnails are way to small.

How can I make them bigger?

Changing the size of thumbnails unfortunately is not (yet?) implemented in the Gtk File Chooser used here. Unless the application in question does not use another file chooser there will be not much change to this, I'm afraid.

From Bugzilla bug #141154 there are some patches around now. Launchpad bug #137606 has state in progress.

While this isn't directly related to the file chooser in Gnome, you can alternatively open a file browser to your Pictures folder and drag/drop into the composing email. This will allow you to zoom in with all the full functionality of the file browser. Evolution is fully drag/drop compliant.

It's control and scroll up on your mouse or control shift =/+

This is because evolution isn't packaged with it's own file browser (like most apps), but uses Nautilus (or whatever your default is, again, like most apps). This is done for unity in your interface and because programmers, in general, are lazy.