As said in this page,
wp_get_attachment_image_src()only accepts built-in sizes such as thumbnail, medium, large or full.
How do you get the same return value with custom sizes you add with
You are misreading the Codex.
Proof of concept:
Add an image to the Library, then...
You will notice that the image returned is (an appromixation of) 300x9999-- that is 300 wide by whatever height scales correctly.