2 liter units is made with 1/2" pipe
 

Pingman, One of my 2 liter units is made with 1/2" pipe. I just got fittings that necked up to 1" for the bottles. If you go with 3/4" pipe you can forget about using the screw cap. You may have to file down the bottle threads as well. You'll have to use a bunch of sealer to get it leak proof. Hot glue might work.
CM

I used 3/4 inch pipe when I built my first 32 plant garden 10 years ago and that same year I built a 2500 indoor garden for a client using the same 3/4 size. First by grinding some off the bottle, then by heating the tee and cramming the bottle on. Both ways took a lot of silicone to stop the leaks.
But when I built my own commercial garden I change to 1/2 inch to save money.
Then a couple of years ago a fellow member brought to my attention that a bottle cap would fit into a 1 inch tee if you use a schedule 26 PVC piece of pipe. No more leaks. The extra cost of the larger size well out ways any savings you would get by using the smaller sizes.