Does anyone know what type of CMOS transistor technology is used in BCM2837 (Raspberry Pi 3 B) ? Especially it's would be great to know what is the size of CMOS transistor (in nm).
That is not strictly accurate. 40nm feature size does not mean the transistors are 40 nm.
It's unlikely that they only used one transistor size. The smallest transistors will be in the memory array and they're typically smaller than any logic transistors because the array is very regular and they can optimize topology for any process node.