These are dimensions that have been tested and proven to work. There is no need to oversize things when it's not needed. Now you can certainly vary dimensions but these are what works for most folks. At least that's what I have read over and over again. Obviously the bigger the tank and the bigger the reactor needs to be since you'll have bigger pumps with stronger flow. The whole idea is to slow the flow and break bubbles small enough so that mixing with water is optimal. How you do it doesn't really matter. These 2 designs Rex Griggs and Cerges do exactly the same, just with different hardware.
Yeap that's good. The further back you put the Co2 the better. Here Tom's Barr way:
Dual venturi DIY External CO2 reactor
And here is the method I used:
Aquatic Plant Central - View Single Post - DIY Inline reactor plans
(the post is not from me but I did the same)