If you only want the resizing to apply to selected shapes, you could rewrite it like this:
Code:
property pnumBaseSize : 20
property pnumDeltaPerLine : 20
on run
tell application id "com.omnigroup.OmniGrafflePro"
tell front window
set lstSeln to selection as list
repeat with oSeln in lstSeln
if class of oSeln is shape then my SizeByIncoming(oSeln)
end repeat
end tell
end tell
end run
on SizeByIncoming(oShape)
tell application id "com.omnigroup.OmniGrafflePro"
tell oShape
set oLines to its incoming lines
set numSize to pnumBaseSize + (pnumDeltaPerLine * (count of oLines))
set size to {numSize, numSize}
end tell
end tell
end SizeByIncoming