Umm, maybe 5' long 1/2" wide.  For the darts skewers with paper cones  or as ben-banned suggested bike spokes.  For blowing stick you tounge out blow really hard and pull your tounge back in.

I would make it as long as you can before it bends. That way you get nice power. The pipe diameter is tricky. I like small diameter, because then it is powerful with light darts, but if you want to shoot broadheads or big darts, you'll need a thick blowgun. As for shooting the thing, I would deeply inhale with your arms over your head because that way your lungs expand more and get more air, and then bring them down and shoot. For the end of the dart, make it snug but not too tight, because if it is tight then after a lot of shooting, the area right under your ears starts to feel strange, like you just blew your nose way too hard. Ouch  

A great man in Blowgunnorthwest forum suggested in order to get the maximum power:
 
- 3 feet for .40calibre
- 4 feet for .50calibre
- 7 feet for .62calibre

not a blowpipe user myself - but surely longer and thinner would be better. ie: total volume of air needed would be less on  a smaller calibre pipe so you could blow out harder and get higher velocity in the dart.  
The same principle works in guns so presumably it applies to blowpipes.  
 

Today I bought a plastic pipe and made a real blowgun! It's (inside) 15 mm, that's quite much I think, but it works. I also practice with the copper pipe.
I put together 5 newspapers to shoot at, and I'll make more darts tomorrow, tonight I gotta do homework...been busy teaching fiddle, and tomorrow I'll also have fiddle pupils, so gotta do the homework today...
The dart I first made is stucked in the roof over the bottom of the stairs, where it's highest up, but I've got lots of bamboo sticks for grilling, they're just perfect for making darts...
 
Gard
 
P.S. This is my post number 100!!!

Hi Gard. I use a bamboo (actually rivercane) blowgun of traditional Cherokee style about 8 feet long. I do demos with it at Powows and really surprise people with the power of the darts. Using just bamboo and thistle dart I can imbed a dart into a tree almost a full inch from 30 yards away I am really interested in those blke spokes though.

My .50 cal blowgun is a 4.5' length of grey PVC tubing, that I useto shoot sharpened paperclips and bamboo skewers with great force. I actually put a piece of wire coat hanger in my wall about 1/4" deep from across the room without even a full force blow. To practice for moving targets, I set a plastic bag up on one fo the blades of my fan with a very light weight in it, and turn the fan on medium power. As the fan turns, the bag moves, and I have to time the shot so it intercepts the bag while its facing me, or the dart goes into the light or the ceiling or the wall. I've gotten good enough to strike the same bag about 8 times in a row out of 10 from the length of the room. Not that it does a whole lot, but its fun!