Measure the base. If the base is wider at the bottom than at the top, use the bottom measurement. If the base is a circle, measure the diameter; then halve it to find the radius. If the base is a square or rectangle, measure its length and width.
Find the area of the base. If it's a circle, the area is equal to pi times radius squared, so multiply 3.14 by the radius (the distance from the exact center of the circle to any point on the edge of the circle), then multiply by the radius again. For a square or rectangle, multiply length times width to get the area.
Multiply the base area by 3 to get the maximum table top area you can use. Choose a glass top with an area no larger than this measurement, because the base must support at least one-third the area of the glass top. Find the area of a glass table top using the same methods as in Step 2. If the glass is an odd shape, such as kidney, measure the length and width at their longest points.