Set The Size Of An HTML Input Number Field

TL;DR

I'm using this to set the size of number input fields.

Code
-- css

.number_inputs {
  width: 3ch;
}

Details

Chrome is automatically sizing form input number fields for me if I put both _min__ and _max__ attributes on them. Firefox is not.

The _size__ attribute also doesn't work. Instead I'm using the above CSS to set the size using the _ch__ unit which is the width "O" character for the font.