why can it handle something at a meter but not a millimeter?

This is a technical decision that was made very early-on during the design of the software.

John Bacus (one of the earliest people to work on the development of SketchUp) said that at that time, they thought people would use SketchUp only to model stuff on a building-kind-of scale, not small objects with sub-millimeter features.

It comes down to how computers work. Computers can only represent a floating-point number of a certain size.

Here is a simplified representation. Let's say the biggest number your computer can store and represent is:

`99999999`

And it can have a decimal point anywhere in there, for instance

`999999.99`

or

`99.999999`

You can see that if you want a big number, it can't go very far after the decimal point. But if you want a number with a lot of decimal places, it can't be very large.

So the SketchUp team decided to have the smallest dimension of 0.001", so that you can have very large dimensions in your model.

The consequence is that if you have an edge smaller that 0.001", it will collapse to a single point.

To be honest, I think this is a big mistake. SketchUp uses `double` numbers, so the maximum size of a SketchUp model is about 25,400,000 km (~½ the distance from the Earth to Mars)

They could have allowed for a much smaller precision and we would still have a very large maximum size. (eg 0.00001" -> 254,000km)

But IMO the best solution would be to have a setting to let the user decide where to put the decimal point, so that we could either model a cell, or the solar system.