What's the smallest thing?

I am not going to say, but it still satisfies my wife.

my checking account that is getting small every month

Who knows and can guess why ?

A blood cell, you can barely see one.

A crumb from a dropped food particle

We don't know. At the sub-atomic level, the word "size" loses its meaning. you cannot put a ruler next to a proton, neutron, electron and see how many picometers or femtometers it is. We can't even tell where such a particle begins or ends

Your bank account.........

Bactria. It is so small you need a microscope in order to see it. 🦠🧐🔎

You tell me.......

Trump's IQ, I imagine.