I have 2 LoRa SX1278 Ra-02 modules connected to Nano 328P boards. I wasn't expecting any of them to work because of the Nano not being able to provide enough current to the LoRa modules, however somehow the transmitter works correctly (the receiver instead doesn't).Questions:1) What's the smallest board I can use "as is" with the LoRa (e.g. without providing external 3.3V power to the LoRa) as I would use it with the UNO? For instance, I don't understand if I can use the mini pro (8Mhz, 3.3V) or not.
if you are looking for a small footprint, (nearly) ready to go solution, consider the TTGO LoRa32 http://s.click.aliexpress.com/e/c8lG1lyy - which brings MCU, LoRa and a Oled on board. Add two wires, connect power, upload sketch - works.
I've used SX1278 LoRa modules with Pro Mini without any issues, your problem with receiver not working is probably not due to power issues. Though it's hard to guess without schematic and code.
As per 2: you might be confusing power voltage level and logic levels. Most SX1278 modules don't have built-in level shifters, so I highly recommend using those if you want to use board with 5V voltage. No need for level shifting with 3.3V with Pro Mini though.
A 3.3V Pro Mini will drive a SX1278 SPI LoRa module just fine.
How do you know the transmitter is working and the receiver is not working ?
I agree with srnet, the article you linked to as the source of your wiring doesn't use Ra-02 modules, instead, it's using Adafruit SX1278 breakout. The Adafruit breakout board has built-in level shifter, Ra-02 doesn't. Add the level shifter and you should be good to go with Nano (assuming the SX1278 wasn't damaged already).The reason I mentioned Pro Mini is that the power output is even lower than on Nano iirc, and it already has 3.3 V logic, so you don't need the level shifter there.EDIT - Oops, didn't notice the article is using Ra-02 IRL and only using the Adafruit breakout in the Fritzing diagram. In that case, whoever wrote it has very little idea what he's doing.
I'm late to this thread, but thought I'd offer my input.