I have noticed quite alot of clocks being sold without their pendulums in some of my local antique malls.

Question: How do you determine the correct size, weight and style for a paticular clock?

Bruce,
I have often wondered what happens to all of the missing pendulums. There must be, somewhere, a bone yard full of the carcasses of rotting pendulums. The next question is why do they get separated from their clocks?
I will try to answer your question as simply as possible. Every clock, when keeping correct time, will have a pendulum that oscillates at a specific number of beats per hour (BPH). That number can be calculated by counting the number of teeth on the clock wheels. (Really only the teeth must be counted from the minute arbor to the escapement) A pendulum with a certain length will oscillate at a set rate. For instance, a pendulum that has a rate of one oscillation per second will be .9939 meters in length. So—It seems that one only needs to know the rated BPH, go to a table, find the appropriate BPH and then obtain a pendulum of a certain length. Not!!! Pendulum length tables are derived from the physics of pendulum operation. Since a pendulum in physics has only one dimension (length) and is devoid of things like friction, the process becomes a little complicated. In reality, the pendulum length tables do help in fitting a pendulum, but normally final adjustments must be made by adjusting the physical length of the pendulum, coordinating the hands of the clock with a known accurate timepiece. This adjustment is usually made via a nut below the pendulum bob. With a pendulum that is governed by the real rules of nature, effective length is of utmost importance. Effective length is the distance from the pivot point of the pendulum to somewhere near the center of the pendulum bob. Weight, within reason, does not have as much to do with the rate of oscillation. In reality, the style pendulum should be one that does not change length with, for instance, temperature change. Metals have actually been developed for clock pendulums with that in mind. A place that one can find a pendulum length table and all of the formulas involved is in “The Modern Clock” by Ward L. Goodrich. This book was written about a hundred years ago. I guess mechanical clocks were still modern then.
Best Regards,
Dick Feldman

Dick has posted some very good and accurate information. To determine style, however, the best thing is to find a book with that or similar clocks by the same maker.

If the pendulum shows through the glass, this becomes an important point to get the look of the clock correct.

If it is hidden inside, as in the case of many mantle clocks, the correct bob is less critical.

A good guess (in the absence of tooth count and length chart) to begin with, is to fashion one that looks right in the case. The bob was always near the bottom. This may take some trial and error, but it can get you to the right length with some patience.