About the dimension of cooked thin spaghetti & maybe a little over an inch long. The point is very sharp so you only feel a brief moment of pain. Just as if someone was pinching the inside of your arm, hard. (Try it, then you'll know how it feels.) The pinch only lasts a moment. I never look until after it's inserted & taped in place. No biggie, really. (Good for you for considering donating. Hope you do, does a lot of good.)
When giving blood the needle is usually between 1-2 centimeters, depending on how you react to pain it doesn't hurt and after the second or third time you won't really feel it. Take it from an experienced donor, you'll be fine and it won't hurt too much the first time.