make icon smaller in list view mode

The icon size can be changed in two ways: With the keyboard using Ctrl-plus and Ctrl-minus or with the mouse/touchpad using Ctrl-Scroll (this also works in the other views). Whatever size you have when you close down will be restored when you reopen Files.

To check your zoom levels in list view run:
$ gsettings list-recursively org.pantheon.files.list-view

You will see something like this:
org.pantheon.files.list-view minimum-zoom-level 'smallest'
org.pantheon.files.list-view maximum-zoom-level 'normal'
org.pantheon.files.list-view default-zoom-level 'smallest'
org.pantheon.files.list-view zoom-level 'smallest'

Then you can set desired zoom with following command, adjusting the key (zoom-level, default-zoom-level, maximum-zoom-level, minimum-zoom-level) and value (smallest, smaller, small, normal, large, larger, largest) to your taste:
$ gsettings set org.pantheon.files.list-view zoom-level 'small'

For informatation, there are two extra zoom levels now between larger and largest: - huge and huger. Levels above "larger" attempt to load a large thumbnail rather than a normal one and are intended for icon view. There isn't a smaller icon size than "smallest". The preset minimum for list view is "smaller" and for column view it is "smallest". So you could make the list view a little more compact by changing the minimum setting to "smallest".