I am interested in starting my first Arduino project. I need to be able to connect to an Arduino board through bluetooth (emulating a serial port) to drive a relay that carries no power, just closes a circuit when activated, opens it when done. What components should I use if the project is to create something as small as possible to do this?
suggest an ESP32 with a 3.3V relay
the ESP32 supports WiFi, Bluetoth Classic and BLE giving you a good choice of technologies
Ok,. Which ESP32 board and relay module is good for a beginner (in Arduino, I have ~50 years experience in IT and programming)? I have never done any Arduino dev, though I do have the Arduino IDE installed.
if you search on EBAY for "ESP32 NodeMCU WROOM32 38 pin" and "3.3V relay" you will get plenty of links
the Arduino IDE supports the ESP32, see installing-the-esp32-board-in-arduino-ide-windows-instructions
if you have programmed in C or C++ the ESP32 should be no problem - they are plenty of libraries and examples to help with WiFi, Bluetooth, connecting sensors, etc
I see the boards with many exposed pins on the bottom. Is a breadboard necessary?
No.
You can simply fit these pins into a strip socket and then solder that socket onto strip board. That is how I do virtually all mt projects.
Thank you, that helps. 
for prototypes I tend to use a breadboard, e.g. ESP32 Connected to a SSD1606 LCD (also ESP8266 and other modules)
however, jumper cables are unreliable and for permanent connection use a solderable strip as suggested by @Grumpy_Mike
This is the smallest Arduino compatible board with Bluetooth that I can think of
It's called a XIAO nRF52840.
Can the relay be solid state? Given that it carries no power, I guess this doesn't matter, as long as it is called a "relay"?
There are some quite small ones like this:
If it must be a mechanical relay, there are reed relays like this one
However, the coils of such relays may need 5V and more current than the Xiao's pins can provide, so a small transistor might be needed to drive the relay coil.
Hmm. That Omron relay looks really interesting. I think I will also try that one. Thanks!
So I have my prototype design working and the code works perfectly. How do I turn this into something that is not a development system, and for which people can buy production parts, housing and wiring to do the same?
I think the first step would be to produce a PCB.
This is not as easy as it sounds because once you do you loose most of the flexibility you have at the moment in terms of what components / modules you can use. The production life time of the parts you use, especially the multi pin modules is uncertain.
But once you have a PCB you can make an open source project, if that is the direction you want to take.
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