Hi,
it would have been nice if you would put a bit more content into your question.
Did you take a look into "skinparam ParticipantPadding"
http://www.plantuml.com/plantuml/uml/PSp13e8m3CRnUv-YuMx2IBnuCCZ1gxp2O8isTki2NSBZksWKrEk__Ntjh7RI71ZcEyLa9nkmBuL6IZPgRvsZUCLzlRbtihoxgG46MR0wYvWAqYhnn3HwyvyQyD5XGH_OVCDE1lCRLlIQnTtn-OS7l7bcsUHBrb8bAxJ5vC0F
Is this what you were looking for? If not, please provide more information to your question.
BR,
Michael