What is the size of the holes in the metal plate of the laser bed?

It depends on the situation.
The hole must be as large as possible to minimize tempering and facilitate smoke exhaust, while also being small enough to ensure that the smallest object cut at once (without using a stopper) will not tilt or fall off during the cutting process.

There are various sizes of perforated plates available on the market to choose from. If there are already cutting tools, I would choose this one. Although it has many holes, it is still very stable and has good flatness.