Here is a reply from Victor at Norcal Brewing (our partner on the false bottoms for our kettles) to a guy who was asking what sized kettle to buy...
The vertical position of the thermometer will not be a concern with a 5 gallon batch unless you are making a low gravity beer and using the largest (20 gallon) brew kettle.
The kettle geometry of all 3 of the SS Brewing kettles is such that with a 1-1/8” stand under the false bottom you have an optimal amount of dead space below, and a good height-to-width ratio above, the false bottom.
Whether you are making a 5 gallon batch or a 10 gallon batch it is ideal to have the width of the mash grains approximately equal to the height of the mash grains. Since the kettle diameters are 2 inches apart (13-5/8”, 15-5/8”, 17-5/8”) the largest kettle is OUT for 5 gallon batches for sure, and I wouldn’t recommend it for the occasional 10 gallon batch (although it WOULD work).
The smallest kettle is best for 5 gallon batches, but won’t work for 10 gallon batches – unless you ONLY brew low gravity beers – so I wouldn’t recommend it.
When making 10 gallons of beer the 15 gallon kettle is perfect. It will accommodate even a high gravity (30+ pounds) beer. You can even use it as a boil kettle to boil down 13.5 gallons of beer to 10 gallons (90 minute boil for roughly an 8% beer) if you are careful of the hot break. You should even be able to make 11 gallons of a lower gravity beer in this setup, no problem (perfect for splitting into 2 x 5.5 gallon fermentation vessels, and eventually into 2 x 5-gallon corny kegs) – 60 minute boil - again if paying close attention for boil-over during the hot break.
When making a 5 gallon batch of an approximately 5% beer (figuring on the 10-12 pounds of grain mentioned) the width of the grain bed will be wider than the height, but that won’t be much of a problem as long as you don’t over-sparge (fly sparging). Batch sparging won’t be a problem no matter what. Over-sparging using fly sparging can produce tannins. In either case, use a refractometer if you can, and stop collecting wort if the reading falls below 1.009 gravity.
Hope this helps!