Flow rates by gravity at 1 meter height is printed on all of the needle wrapper. Most manufacturers have it on their websites too.
For example, for BD Angiocath:
14 ga 1.75" = 330 ml/min,
16 ga 1.77" = 205,
16 ga 1.16" = 220,
18 ga 1.16 = 105,
20 ga 1.16" = 60,
These numbers are all for gravity flow, under pressure they will be much more. A "Cordis" introducer comes in different sizes, most are 8.5Fr or 9Fr usually 10cm. 9 French is 11.2 gauge or about 2.3mm internal diameter vs a 14 ga (1.6mm ID). RICs (Rapid Infusion Catheter) is usually a 8.5 French 7cm catheter made for the arms (there's also a Cordis 7cm one). So you can see the difference can be dramatic all based on the Hagen-Poiseuille equation.
What's most important is performance under pressure. A 14 ga catheter can flow about 500 ml/min at around 170mmHg, which isn't bad. At the same pressure a 9 French introducer can do at least 750 ml/min. I've done 1.5 LPM on a 9French introducer during venoveno bypass for liver transplants. Lower pressure is prefered to avoid hemolysis. Nurses in general love to put caps on big IVs, which produce ~30% reduction in flow.
One more thing: peripheral IVs are much more likely to extravasate than a central introducer. Pressurizing a blown peripheral IV is terrible, as I'm sure you've all seen.
Of course, the world's best IV is an arterial cannula in the aorta.
