Right now it's not very clear how is computed the width a standalone macro occupies in the WYSIWYG. There's no CSS rule or DOM properties for that, it's let to the browser implementation. For example when having a standalone RSS macro, the macro block takes about half the line width, and the content of the macro (the RSS div) takes only 30% of that half (that's default setting of the RSS macro), so it does not look good in the end.
To be safe and have a clean display everytime we should make all standalone macros use 100% of the line. The plus with that is that users understand they cannot add text or anything around the macro, since it's a block.