How to show full names on desktop in XFCE?

Have a look at the HIDDEN CUSTOMIZATIONS mentioned in xfdesktop's README:

If you're using the icon view, and would like to change how the text looks, you have three things you can change: the opacity (transparency) of the rounded text background, the color of the rounded text background, and the color of the text itself.

You'd want to add something like this to your ~/.gtkrc-2.0 file:

style "xfdesktop-icon-view" {
    XfdesktopIconView::label-alpha = 75
    XfdesktopIconView::selected-label-alpha = 100
    XfdesktopIconView::ellipsize-icon-labels = 1
    XfdesktopIconView::tooltip-size = 128

    XfdesktopIconView::shadow-x-offset = 1
    XfdesktopIconView::shadow-y-offset = 1
    XfdesktopIconView::shadow-color = "#ff0000"
    XfdesktopIconView::selected-shadow-x-offset = 2
    XfdesktopIconView::selected-shadow-y-offset = 2
    XfdesktopIconView::selected-shadow-color = "#00ff00"

    XfdesktopIconView::cell-spacing = 6
    XfdesktopIconView::cell-padding = 6
    XfdesktopIconView::cell-text-width-proportion = 2.5

    base[NORMAL] = "#00ff00"
    base[SELECTED] = "#5050ff"
    base[ACTIVE] = "#0000ff"

    fg[NORMAL] = "#ff0000"
    fg[SELECTED] = "#ff0000"
    fg[ACTIVE] = "#ff0000"
}
widget_class "*XfdesktopIconView*" style "xfdesktop-icon-view"

According to this XFCE forum post,

XfdesktopIconView::cell-text-width-proportion = 2.5

should be what you're looking for (try a higher value).

Edit Reading more of the thread I linked, I gather that

XfdesktopIconView::ellipsize-icon-labels = 0

should do a better job (that's already in the README excerpt... well, I didn't notice it).

In my home directory, I had to create a file called .gtkrc-2.0 as there wasn't an existing one. In this new file I entered and saved the following:

style "xfdesktop-icon-view" {
    XfdesktopIconView::ellipsize-icon-labels = 0
}
widget_class "*XfdesktopIconView*" style "xfdesktop-icon-view"

Nothing else was needed in that file, just the code as written above. When ellipsize-icon-labels is set to 0, there is no truncating of the filenames. When it's set to 1, then there will be truncation.

When you've saved the above, just log out and back in again for the changes to take effect.

This answer is based on the README as found at the xfce git repository.