If you ever see one in stores, you can differentiate between the sizes available by looking at the light blue box on the upper right side of the plastic packaging. The characters you have to look for are 細字 for Fine, 中字 for Medium, and 太字 for Large.
The fude tip is almost identical to the Kuretake Cocoiro Super Fine tip, but I actually find that the Zebra Fine Pocket Brush Pen works better for me at less than half the price of the Cocoiro! Awesome, right? I still love my Cocoiro to bits, though!
The Zebra Pocket Fine Brush Pen is perfect for those with heavy hands like me who have trouble producing fine lines in calligraphy. This brush can easily create super fine (0.05MM) to medium lines (1.5MM) but requires a lot of force to create about 2,5MM lines.
Right off the bat, the brush pen works beautifully and I have a feeling I'll be using and repurchasing this for a long long time. :)