The smallest transistors currently in production are the intel 18A which have a gate length of 18 angstroms or 1.8nanometers.

However this is more of a naming convention than actual size as transistors have moved off planar design for a while now as FINFETS and GAA transistors became the norm.

The smallest planar transistors are around 22nm which would give you around 450,000 stacked end to end in one cm.

I however don’t think that’s the core breakthrough here. It’s more about it’s durability seemingly.