I found the was to left-align the name of the column using the following code :
private void radGrid_ColumnChooserItemElementCreating(object sender, ColumnChooserItemElementCreatingEventArgs e)
e.ItemElement.TextAlignment = ContentAlignment.MiddleLeft;
But I did found how to set the height of the elements in the list of columns (see screenshot attached).
Is there any way to do it ?