So many landing here want codified data on fusor construction and dimensions. There are no rules. Operator skill and construction will be the deciding factors.....ALWAYS!
In general, the following will keep you out of big trouble if a spherical chamber and grid are selected for the fusor assembly.
1. Ratio of grid diameter to chamber diameter. Gd:Cd = 1:6 to 1:2.....1:6 is considered ridiculously tiny and 1:2 is considered huge. Do not be confused by this wide range. All of the these ratios have done fusion!
2. Set the chamber diameter in inches, (Cd), to equal ..... Cd=0.15(KVapplied) with Gd:Cd = 1:5. (i.e. 40KVapplied would need a minmum of a 6" chamber and a grid of about 1.25" in diameter.) This equation is for a KVapplied of less than 100kv, which is the normal maximum for amateur work. Arcing will be a considerable challenge as the grid size increases in relation to chamber size, mostly due to the hanger stalk design, assuming correct vacuum.
3. Operation is with a flowing D2 gas against a continuous, balanced vacuum creating a fusor chamber atmosphere of at least a 500:1 or better, D2 gas to air mixture at a pressure between 3 and 15 microns.
There! It has been codified for all the rule follower, lock-step types!
Demo fusors tend to use smaller grid size ratios as they will never fuse and the neophyte will have insulation and vacuum issues to contend with and it is generally safer to use the 1:5 or 1:4 ratio, thus, my example above is aimed at the demo fusor. 1:4 to 1:3 is the current norm in amateur hands that actually do fusion here.
With the above, we assume you are capable, (big assumption), of finding, installing and operating a proper vacuum tight insulator rated for the KVapplied that you are dreaming about. For neophyte operators. The above is easily said and perhaps even digested, but difficult to obtain. Competent operation of the fusor is an acquired skill and always the final deciding factor. Grids that function to the Mega range for one person will just not work for another. (skill and operational standards)
Normal, safe construction usually presents as a spherical 304 stainless steel chamber with proper Conflat, ISO, ASA. KF and Swagelok or VCR fittings. The vacuum is usually a two stage mechanical fore pump, backed by an oil diffusion or turbo pump. Other designs and materials are serviceable but may alter the above 3 major advices.
95% of all successful spherical systems are 6" to 8" in diameter and never exceed 60kv applied except in a rare instances.
The Pro's swear by a 1:2 ratio, but they have pro gear, operate at reduced pressures and in voltage ranges that the average successful amateur will never have.