So I created a sensor array with a DHT11 sensor, PMS7003, and HC-06 for Bluetooth connectivity. And I was wondering whether it would be ok if I connect a 12V 5A power supply to 2.1x5.5mm power jack in Arduino Board?
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One of the most important pieces of information that you can give when posting a question on this forum is the type of Arduino board that you are using
I am guessing that it is a Uno, but am I correct ?
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Yes! I'm using a Arduino Uno R3V2 board!
It can depend on what You connect to the 5 volt pin or the 3.3 volt pin.
If you only power those 3 sensors, yes. If you have plan to add others, better to find another solution, like 7.5V psu on jack or direct 5V power supply. Or power your stuff separately.
Let's thy to give some theory
The 12V on the barrel will be converted to 5V by a voltage regulator.
- That voltage regulator will heat up and if it heats up too much it will shut down.
- The amount of heat is determined by the difference in voltage (12V - 5V) multiplied by the required current. Hence @kmin's suggestion to use a 7.5V power supply; a 9V power supply will also reduce the amount of heat.
- The voltage regulator on an Arduino Uno/Nano/Mega has virtually no heat sink so will overheat reasonably easily.
- The 5A is irrelevant as long as it meets the current requirements of your setup.
Just test how hot the voltage regulator becomes in your setup when applying 12V to the barrel.
I'm not in favour of feeding 5V into an Uno on the 5V pin; you might have a conflict with the USB power and possibly damage more than likely the most expensive component (your PC).
I use a 5V supply and a boost converter; 5V in, 9V out to the Vin pin. And the 5V to all 5V modules.
For curiosity, why 9V instead of 7.5V?
Not living at the edge 
I have 9V ones (non-adjustable) available.
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No if you have all those items connected to the 5V output.
+1. There's USB, VIN and the barrel jack. I don't get why anyone would favor using the 5V pin to power the Arduino.
Not ideal, but sometimes it's the quickest way. If you have some powered external hardware with 5V output for example.
Or for example I have good quality 5V10A psu to power neopixel leds, I connect it to those two "thicker" wires. Then I need to connect it to arduino through smaller wires with connectors. gnd to gnd pin, data to gpio pin, and.... ?
Separate Usb supply to arduino?
Or the last neopixel wire vcc to 5V pin...
Call it laziness .
I still wouldn't recommend it, because stability of your arduino now depends on that external hardware and you need to remember to disconnect 5V if you want to use Usb.
Edit: ws2812b led strips, not just neopixel
My magic blue smokeless Speakjet and TTS256 ICs would like to ask you about this "quickest way" concept... **
Hey, if it works for you, it works for you. It's better to have the option than not, I suppose. If I ever make something using more than 16 NeoPixels at a time, I might just remember this description and use it.
I typically use higher voltage PSUs (or multi voltage PSUs) and dial in my voltages with buck converters, but that's just me. 
** about the dead ICs: it's ok, I've had time to get over it even though these chips don't seem to be available anymore. I did love that Speakjet though - what a fun IC she was.
Sorry, I called it unproperly neopixel, I meant ws2812b led strips (with 5 wires for connections).
Get a 5V supply with a USB-A connector and plug the Arduino's USB-B cable into that.
Done.
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