Yeah, 21g is pretty big... Back in the States I would usually stick to 29 or 30, 31 made it too difficult to hit a vein. Now here in Portugal I don't know why but they only seem to have (as far as 1ml insulin syringes go) 25g (orange tip) and 28g (blue tip) needles. Can't find any smaller...
At the exchanges it's always the blue-tipped 28g ones. After not touching the needle for over a year and a half, the first time I IVed after all that time was with the orange-tipped ones, didn't even check the gauge, and didn't really notice it was that big, then again I'm used to the sting of the needle poking through my skin...only a few days later when I was buying a syringe again did I take the time to read the size of the needle and I was like "Holy fuck" when I realized it was a 25g point. At first it seemed easier to register and shoot with the 25g, but that shit damages your veins like crazy in no time...
These days (by which I mean, when I actually do IV, once in a blue moon...I am trying my best to do it as little as possible) I stick with the blue-tipped 28g spikes... But for some reason they seem "slower" to push down the plunger and harder to register than when I was using 28-29-30g BD/Terumo/whatever non-detachable insulin syringes back in the US...
With the 25g tips that shit goes down in an instant...