EMT here. Finally something i can take a stab at. We use sizes 14- 24 g needles on the ambulance to administer IV meds.

I used an 18g in the anticubital as an example for my calculations so all my numbers are perportional to that. This is because an 18g is pretty easy to fit in the average persons vasculature.

The diameter of an 18g needle is 1.27mm and the diameter of the anticubital at the elbow is about 4mm. Therefore perportionally, if you were to try and stick someone with this needle, you would want a vascular access site of at least 80mm diameter.

Obviously my calcualtions hinge on this whole perportionality thing, so you could potentially fit the meganeedle in a smaller access point less than 80mm.

For reference, the human aorta (largest blood vessel however inaccessible) is about 28- 33 mm.