The most commonly used sizes are 18g and 20g. Any patient who you suspect may receive blood products, or who is or could possibly be a surgical candidate, should have at least an 18g. Smaller catheters can cause hemolysis of the RBC's being infused. A 14g or 16g is preferable for patients with significant blood loss, as in trauma. Avoid long catheters.
Longer catheters are more likely to cause problems when threading the catheter into a vein.
The hand is usually an easy site for starting the IV, but results in more pain. The antecubital fossa should be avoided if possible. Flexion at the elbow will kink the catheter. It is OK to use the AC when you need a big bore catheter for trauma. The AC is also a good choice for patients in supraventricular tachycardia.