Spacer fields are not true fields, because they do not store any data.  Spacer fields simply add vertical space in the auto-generated "generic layout" of a form.  Spacer fields have very few settings.  Even most of the common settings are missing.  On the main edit page there are only the name, size and custom lookup properties.  On the advanced tab only a simplified locking and hiding setting, and style class are available.  The permission tab, however, is unchanged.  You may be wondering why the permissions and locking an hiding settings are available at all:  It is simply to make the spacer go away when surrounding fields disappear, so you don't end up with a bunch of meaningless whitespace in your layout.

Format/Setting	Description
Name	The name setting is exactly the same as for other fields:  It identifies the field for the form editor's benefit.  However, it is a bit different in what it doesn't do.  Spacer fields are not listed anywhere except the form edit screen.  Thus they have no label and the name is never used anywhere else.  Some designers choose a very simple name for their spacers such as a single period or just the word "spacer."
Size	This is the height of the whitespace in pixels that will be added to the generic layout.  All BlueStep's generic layout spacing is in multiples of 6 pixels.  For instance, the indent of a grouped field is 12 pixels, and the space before and after a header is also 12 pixels.  You may wish to continue this convention, but there is no need to.