Agree with both of the above but I also like to look at how I may want to work with the reactor and how it fits my stand. One point that is missed often is that we do need to clear the water from the outgoing line of a canister as we restart it after cleaning. Some do have a problem on restarts as the air in the canister does need to move out and up the outgoing tubing to get the can to start correctly. The little thing that we all know but may miss is that air WILL go up and out through water but WILL NOT if it has to go down first to get up and out. So if we have a reactor and canister setting on the floor under the tank, the air will not always go down and out if the reactor is full of water. I find I want to use a flat bottom on my reactors, so they set up nice, but I also want to leave them loose so that I can swivel them up to let the air past the water! Different folks report different results on this, so I recommend leaving the design loose for a time until you see how your placement works.
The difference is that the reactor may need to be strapped up off the floor for a straight fitting which does add to flow a bit but may make restarts difficult if you do need to clear the path for the air to move out.
I like the flat bottom from using a PVC plug at the bottom rather than a cap which is rounded and I'm willing to give up a slight amount of flow.
These are reactors that I have built and each has slightly different fittings. The one I liked best was the center as it used an irrigation fitting with a compression type seal. The one at the right needed a metal seal to assure a good connection, but they both do add two 90 bends to impede flow somewhat while the one on the left used a larger curve in the tubing to let the water flow without the second 90. Larger/taller stands let me go with a wider curve.
For size, I used 18 inches for one fed by an Eheim 2075 on a 125 but for 75 gallon tanks a 15" pipe (makes 18" with fittings) was fine for 75 gallon tanks working on 2217 filters. Longer never seems to be a problem if you have room while shorter can let bubbles be forced through if the combo of lots of water moving through and a massive amount of CO2 is added. I have never had that happen except on one where I cut the pipe size down to 3/4 inch for a small tank running on a Zoomed 501 filter. Just too much flow and not enough time, so I got bubbles in the tank. Dumb idea!! But a pretty cheap lesson!
Small point that might help newer folks who do not use much PVC? We often see the purple primer and it get smeared all over. I find no need of primer if I have decently new solvent and pipe. The primer is needed for older pipe that has some corrosion built up or if the solvent is beginning to lose some of it's volatile chemical. It does act to pre-soften and clean the old pipe but we can get a better looking job if we leave it out on small simple joints, done inside where we are dry and there is little water pressure. Rather than use primer, I do a good sanding job as I get the pipe ends ready and try them for fit before adding the solvent.