What is the smallest metal object you can ground yourself with?

If you are installing a CPU in a motherboard, you don't really care about grounding - what you really want to do is to make sure that you, the CPU, and the motherboard (and any tools) are all at the same potential. If you have one of those grounding bracelets, you should connect its ground lead to the computer case or motherboard Ground - whether that is "Really Ground" or not is not particularly relevant.

Connectivity to earth is what matters, not the size of the object.

Size only matters if we are going to get nitpicky...if the wire connecting you to earth is so tiny that it vaporizes when current flows, for example.

In your example, the matchbox car is useless as a grounding object, while the statue is excellent.

The idea is you are looking for a way to remove excess charge from yourself so it cannot discharge through a circuit and harm it. An isolated metal object no matter the size (within reason) is not going to help with this. You simply need a conductive path between yourself and earth.

Edit: Earth is simply a convenient reference. You have access to earth in most places with electrical power outlets. As others have pointed out the important thing is not to have excessive charge with relation to what you are working on. If what you are working on is not connected to earth, you should ground yourself to it instead.

Connectivity to "earth" doesn't matter, regardless of size, at least for the purposes of ESD (electrostatic discharge).

The reason we use grounding equipment is to avoid damaging the parts we're working on, because there is a chance for a rather high voltage difference between you and that part - and the second you touch the part, that voltage equalizes with a ZAP, sometimes letting out all the magic smoke.

What ESD grounding equipment does is help remove that difference. If you're using an ESD table, it will have a resistive top, and a metal point to hook a wrist strap to (though resting your hand on the table may work, as well). When you touch the table, you and the table will quickly reach the same voltage potential. It's not that tables magically drain away energy, or that anything grounded to a big metal pole stuck in the ground will always be safe, but rather that the part on the table and your fingers are tied to the same voltage. If you have your parts in a plastic bucket, they will probably be at a much different potential than you, regardless of what ground you use, simply because the plastic acts as an insulator.

As a note, professional ESD grounding equipment isn't just a wire; it also has a resistor, usually in the megaohm to 1 gigaohm range. Having a high resistance means the voltage potential drains creates a lot less current than a near-zero-resistance wire would. Running a thousand volts through a megaohm resistor makes 1 milliamp (I = V / R, where I=current in amps, V=voltage, and R=resistance in ohms); running a thousand volts through a 1 ohm wire makes a thousand amps. It's only for a split second, but that's enough to do some serious damage to a circuit board!

So to answer your question: the grounding object can be any size, as long as the parts you are working on are also connected to that object, and if you use a resistor rather than a straight wire, you'll limit the damage that equalizing potential will do. Size doesn't matter - if you're working on something size of a grain of rice, well, that Matchbox car may work just fine.

One additional note: ESD grounding and system grounding are not the same. ESD grounding is designed for high voltage, low current static electricity, to safely equalize the voltage difference without damaging a part. System grounding, on the other hand, is designed to direct current from dangerous shorts or eddy currents to an earth ground, away from users. For system grounding, you want a solid connection to true earth, with the lowest resistance possible.