Was wondering what size heat shrink fits around the arms of a mini quad to hold my esc in place on each arm (I have got the QAV210). I've seen a bunch of people using large heat shrink but it's too huge to tell what size it is on photo.

I use electrical tape LoL. Best bet would be to measure the head of the arm where the motor sits and get heatshrink roughly that size

I use 28.5mm heat shrink tubes, but in some cases I've had to insert needle nose pliers and stretch it out a bit before it would fit over both arm and ESC. I've successfully used 30mm heat shrink too but opted to go with the lighter, thinner PVC heat shrink designed for 18650 batteries, 28.5mm.

And just remember, if you've cut away the original heat shrink and exposed the ESC in order to solder battery/motor leads, and your frame is carbon fiber, you'll need to insulate the ESC from the frame because it's conductive. I usually use a strip of electrical tape on the bottom of the ESC to protect it, then heat shrink the whole thing in place.

For heatshink I like using the Polyolefin kind, it is thick and soft and takes a good amount of abuse. https://amzn.com/B0099LHBZI For over the arms, you should get the 20mm or even 25mm. I don't like over the arms because anytime you need to redo, you need to take the motors off. I don't like messing with the motors once I bolt them down with threadlock.



Another option I am starting to favor is the Silicone Rubber Self Fusing tape. It comes in different colors and widths. This one is clear and is great over LEDs - https://amzn.com/B00HWROKAK It is very strong, but soft and takes crashes well. It is very easy to apply over the ESC - pull on it a bit to stretch and it fuses over itself nice and snug. Very easy to take off - x-acto knife gentle cuts along the side of the arm and it comes right off.

I second the self-vulcanizing tape proposal. Getting heatshrink over arms may be cumbersome if you've already assembled everything :-)
P.S. Funny thing, my initial build uses the same heatshrink as Jordan linked to. Going to take it off soon to paint edges with PlastiDip and perform C2-flashing with latest BLHeli bootloader. After that going to use self-vulcanizing tape.