Open symbols are larger than closed symbols even though I chose the same size.

When you plot an open symbol, Prism adds a border (of a width you select) so the overall symbol is larger than the "same size" closed symbol. To avoid this problem, only select "open" symbols. When you select an open symbol, you can separately choose the color of the border and the interior. If both are the same, the symbol looks like a closed symbol but of exactly the same size as an open symbol (if you pick the same symbol size and border width).