Its called open beam, I think min is 20mm, or maybe its 15mm? I’d have to go measure it to confirm that. I haven’t bought any parts for it in a long while. Adafruit stopped selling it after I bought my kit.
I’m currently using the USB port to power my UNO. I will be buying a 9V barrel connector power supply from Adafruit at some point. I have a 16 x 2 LCD screen on my UNO. Its working just fine powered by the 5V via the USB port.
I think you can go as high as 20V in but the 5V regulator will work hard and get hot with anything over 12V. Which is why they recommend 7 to 12 V if you use the barrel jack power in or the Vin pin. .