Pedestal table minimum base size?
The physics of the problem are simple, as the base gets narrower, the table will get less and less stable. This is true whether the table is on four legs or on a pedestal base.
The question doesn’t typically come up on four legged tables because the basic design forces the legs out close to the four corners, automatically creating a stable base.
So to achieve a stable pedestal base the bottom needs to extend out close to the perimeter of the top, especially across the narrower dimension, a 36 inch base width is probably the minimum, but I’d go wider if at all possible.
The length wise dimension can come in some, maybe 12 inches shorter on each end and still be stable, but you won’t be able to come in far enough to leave full clearance to push in a chair from the ends.
The second way to achieve stability is to lower the center of gravity by weighting the base but this usually requires a lot of weight creating other design problems and creating a piece of furniture that can be difficult to move.
Is the design based on a single large column centered in a 78″ by 42″ top? This isn’t a common design because it is very difficult to achieve a table of this style that is stable, easy to sit at, and sturdy.
The typical rectangular table on a pedestal base will use two smaller pedestals using the basic proportions seen in the classic Shaker style dining room tables. Some of the Shaker tables have a narrower base crosswise than I recommended above, but I suspect that they aren’t as stable as most people would desire.
A final point, pedestal tables have to deal with a lot higher stress on the joinery than most other furniture, so a weak design will create a lot of problems that can take the fun and profits out of a job.
John White, Shop Manager, Fine Woodworking Magazine