One of the most important factors to consider when making your blowgun is it's size. A human lung holds around 240-365 cubic inches of air. Some of that air remains in the lungs between breaths though, so actual useable lung capacity is between 100-180 cubic inches for a child and about 220-300 cubic inches for an adult. A rule of thumb for blowgun size is that you want the volume of the blowgun tube be a maximum of 1/10th of your available lung capacity for good dart acceleration.
A 36 inch .40cal. blowgun has a volume of about 4.5 cubic inches. Increase the length to 48 inches and the volume increases to around 6 cubic inches. These are good choices for small children to use for target shooting. This size blowgun can also be found at up to 7 feet long ( 10.5 cubic inches) and is commonly used in paintballing.
A .50 cal. blowgun has nearly 9.5cubic inches at 4 feet, almost 12 cubic inches at 5 feet, over 14 cubic inches at 6 feet and about 16.5 cubic inches at 7 feet. These are routinely used for high end target shooting, or entry level small game hunting.
A 7 foot long .625 cal. blowgun has a tube volume of over 25 cubic inches. These are the high end small game hunting blowguns. The 7 foot long .81 cal. blowgun I made had a volume of over 43.5 cubic inches, well in excess of the 1/10th lung capacity rule of thumb. I got very poor performance from this blowgun and eventually had to shorten it to reduce the tube volume.
Another major design factor in blowgun tube design is tube length. For instance, a 7 foot long .625 cal. blowgun has roughly the same volume as a 4 foot long .81 cal blowgun. But a dart from the .625 cal. will perform much better, largely due to the increased length of the tube. Because of the smaller diameter, and length of the tube, the gas (your breath) remains compressed longer against the dart. Specifically, the compressed gas has more time to exert it's force on the dart, due to the increased length, and the gas is more highly compressed due to the smaller diameter. This principle carries over to conventional firearms as well... the same round fired from 2 different guns will travel farther from the gun with the longer barrel.
The down side to increased tube length is that eventually the tube will not be able to support it's own weight, and will begin to bend when aiming. For 1/2" EMT tubes (.625 cal blowgun), bending is visibly evident at 7 feet.
So, that's a lot to think about when you decide to make a blowgun. Let's assume that we have weighed all the factors and decided to build a 6 foot long .625 cal. blowgun for my wife, so she can hunt squirrels with me (even though she never would).