The smaller the number, the bigger the needle. For example, a 24 gauge needle is much smaller than a 18 gauge needle. With children the needle will usually be smaller, because their veins are smaller.
The larger the Number, ie 25 gauge vs. 21 gauge the SMALLER the lumen or "barrel width" of the needle tube. Think of it as a pipe and the distance that measures across the diameter of the opening,,,the larger that width, the greater amount of a substance can pass through. In needle gauges #'s, however....think of opposites: # 19 gauge is "wider" than a # 25 gauge, which is much smaller, used for subcutaneous injections such as insulin. A # 23 gauge needle is the standard for an IM injection, but it's lumen is narrower than say a #21 or a #19. A #23 is usually used to draw blood, unless the patient is a difficult stick, in that case, the "butterfly" a # 25, will be used, usually on the hand. A liquid medication such as B-12 or testosterone is a much thicker medication, so sometimes a smaller "#" on the gauge is indicated, especially condsidering the area where you are shooting....A deep IM injection on the glut area (buttock) on a person that has a bigger layer of body fat may 1) require a longer needle to reach the muscle and 2) a smaller gauge needle which is a "wider" needle" ie gauge # 21 ) to get the med into the system.
"Needles" are not always used invasively.
Many large bore needles are used to infuse a solution into an IV set through an elastomeric seal in the line.
A smaller gauge needle will mean a longer time for an intravenous solution to enter the vessel, so if a rapid infusion is required, a larger bore is desired.
Bear in mind that when an intravelous line is started, the "needle" is really what is called a trochar, and is withdrawn after the vein is entered, leaving only a plastic tube within the vein.