"side of a salad"; haha, you'd meant "size of...".
it don't sound particularly crazy. just out of the norm.
a "natural" explanation may be something involving "afterimage"...
you know, where you stare at something for a time, then look away.
was there any other aspects for the phenomena?
like, was there a noise or a "feeling" involved?
did you sense any sort of "communication"? (other than: "that's odd")
if it was a one-off thing, you can put it out of mind i reckon.
if it reoccurs,
then you'll wanna truly investigate i suppose.
no worries, it sounds benign.