An insertion in a red-black tree, as in most binary trees, happens directly at a leaf. Hence the size of the subtree rooted at the leaf is 1. The red node does have two black children, because leaves always have the "root" or "nil" as a child, which is black. Those null elements aren't nodes, so we wouldn't count them.
Then, we go and adjust the sizes of all parents up to the root (they each get +1 for the node we just added).
Finally, we fix these values when we rotate the tree to balance it, if necessary. In your implementation, you will probably want to do both the size updates and rotations in one pass instead of two.