Twice the source mean radius; body diameter only, excluding rings.
Put Venus beside Earth and the two outlines are remarkably close. Small differences in diameter are easiest to notice when the objects share a scale and a baseline.
Similar size is only one comparison
Matching silhouettes do not mean matching environments. This tool deliberately isolates size. Use “How many across?” to see how close the diameter ratio is to one, then compare Venus with Jupiter for a very different perspective.