Everybody will have their own ideas. For me, I love symmetry, so my setup is always the same - two identical turbines, and sandwiched in between them is a 3-block-wide Big Reactor. The width matches the width of the Turbines (I always go for max size Turbines which is 16x9x9). The height is really up to you and based on experiment. I learned that a 7-tall reactor can easily run two 9-tall Turbines. So with a 3x7x9 Reactor, that leaves us 7 columns in the interior. I alternate fuel rod-gelid cryotheum, so that's 4 columns of fuel rods and 3 columns of gelid in-between. Even tho a 7-tall reactor works, I usually just go with 9-tall (that symmetry thing!). But the exact height really doesn't matter, as you will see. We can adjust to anything.
Now then, once you get THAT all set up, you're still not done yet. Now you need to balance your reactor's output with the turbines' water/steam requirement. Here's how you do that. Start everything up (make sure you have enough water supply to the reactor obv, I use a Liquid Transfer Node with a stack each of World Interaction and Speed Upgrades with the Node sitting just above the middle of a 3x3x1 still pool of water). You will see the reactor heat go right to the max - that's bad. You're wasting a lot of fuel. So now go up to the Control Rods on the reactor, and use the key to change all of them at the same time (CTRL key, or maybe ALT). Reduce the Control Rods to some setting - start with 50%. Now go down to the Reactor Controller and see what the heat stabilizes to. For me, I want the heat to be around 500 degrees - enough to provide 4000 steam to the two Turbines (2000 mb to each). Keep lowering or raising the ctrl rods in small amounts and keep checking the Controller. Eventually, when you insert the Control Rods TOO far, your steam supply will become unstable - you will see the reservoir of steam in the Reactor begin to fall down to zero. This is bad. You went too far on the Control Rods, but now you know you're in the right ballpark. So keep adjusting by 1 or 2 until you find the perfect setting where the steam is stable but the temperature is low (and hence, you're burning the least amount of fuel possible to generate 4000 mb of steam).
Hope that helps!
P.S. You can easily do this method with just one Turbine. I always begin with just one, because it's a TON of materials to build two (or even just one LOL). But I build with the anticipation that someday, I will be adding that second Turbine. The only difference with one Turbine is that you will need to insert the control rods a lot more (say, 70%) to keep the reactor from operating way too high temp and wasting a lot of fuel.
P.P.S. The beauty of sandwiching the Big Reactor in between the two Turbines is that I can butt up the Reactor Steam Ports with the Turbine Fluid Ports - make one pair each red and butt them up to one another, and make the other pair blue and do the same. Then match this on the other side of the reactor for Turbine #2. Easy, simple, neat and symmetric.
