Pendulums are available for most types of clocks and the correct length is often stamped on the back-plate of mass produced clocks. American clocks are in inches, German in mm, and French in lignes.
For French clocks, you may find a number such as 4.2 on the backplate. Multiply the first number (the 4) by 27.02. Multiply the second number (the 2) by 2.256. Add them together and you will have the pendulum length in millimetres.
The length is from the uppermost flexible part of the suspension to the centre of mass of the bob. In practice, this is the centre of the bob, but in theory it is a fraction higher.
French clocks usually have the movement serial number stamped on the pendulum. If there is any discrepancy in this number and the number on the backplate, and the clock cannot be regulated, it may not be the original pendulum.
Pendulums for French clocks vary in weight and size, and there are three standard types available. There is no specific rule on how to identify the type needed, but generally speaking:
Size 0, the smallest, is used on clocks with no striking mechanism.
Size 1 is normally used on standard striking clocks.
Size 2, the largest, is used on large movements. These are often found in large cases, with large dials. There is no accurate way to easily determine which clocks take the large pendulum, however a movement with a back plate of 100mm or more probably does.
Portico clocks had much more ornate pendulums that were visible.