Game Tables

Small battle with fast results; if true 15mm scale rules are used, e.g. not necessarily 4 inch hexes, table size is sufficient for fairly large battle. However, even though 15mm figures exist, it is common to see games that have linear scales almost identical to 20-25mm distances. 15mm, although originally intended to create a smaller playing surface, almost always uses the same size surface as larger scale games.
Should not be made of anything less than 1/2 inch plywood unless you use some cross braces. CDX 5/16 can be used if you build a good frame to screw it to.
Most sheets of plywood and other materials come stock in 8x4ft in length from the factory, so all this means is that you have to cut 2 feet off one end.
In some cases, you might consider just leaving the table uncut, using the sheet as is.

Requires only 4 legs or can be put on top of any existing table.

Requires two tables to be put together. Each table is 4x6. Possibly requires 6 or more legs and at least one, a double, or a 4x4 "rail tie" leg to be placed in the middle of the table supporting the seam center. This is to prevent sagging and increase sturdiness.
Tactics begin to be affected by the size of the gaming surface.
Cannot be placed on top of any existing table unless the table is already large.
Probably best to create a huge frame of 2x2's and attach the game surface to the 2x2 frame, then set the frame onto the leg assemblies.

Most practical game table for large battles, heavy, and items in center are getting hard to reach unless table is lower.
Probably best to use 4x4's for all legs and center beam supports. Usually constructed from four 3x5 foot size plywood sheets.

Largest game table, heaviest, and hardest to reach the center of the surface. Probably best to keep such a large table lower to the ground to make reaching the center of the surface easier.
Usually results in battles avoiding the middle regions of the width during combat. So middle would be for eye candy terrain.
A sturdier structure can be obtained by applying struts between legs. Under-surface shelving might be interfered with.

Hinging table surfaces together, or hinges used anywhere for any substantial manner or length or support intention, are not recommended. If fasteners of the hinge are not deep enough into the surface or material, the hinge can be compromised. However, hinges CAN have value for non-vital components, for instance, leg braces. Avoid whenever possible.

Add hills, lakes, rivers, trees, and structures. Keep in mind that if you make a flat table, you can always change it with terrain. If you make anything permanent then you stand a chance of getting bored with the table. So keep all your pieces of terrain as mobile as possible. You can also make many layers to a table. With foam you could have a valley floor, or river as the table top. Also a layer of foam with CDX as the bottom can be a whole terrain piece covering much of the table. So it can be removed and replaced with new terrain as needed. The foam as a hill or mountain, and place the other pieces as you need them. I will try to incorperate some pictures below when I can.