MxBluEI got the 0.4mm and coming from the regular Bic pens I really think these are already on the super-fine side. I wouldn't go lower than 0.4 IMHO. It all depends on what you're used to write with.
MxBluEAnything smaller than .4 will be a different style than you're likely used to. You may have to hold the pen at a higher angle or learn to use less pressure. That said, I love them all and have a mix of tip sizes that I change based on my mood.