i think u didnt put ur cotton correctly la ..

Exact same problems i had. It is your wicking style. I have fixed mine by separating the cotton into both sides of the juice hole. Don't use too much cotton.

around 4mm, and cut the cotton short so that the edge reaches the base of your deck (rba).

cw-20 stand for 2.00mm inner diameter

cw-25 stand for 2.50mm inner diameter

cw-30 stand for 3.00mm inner diameter

only difference i know is the size    .. i love to use 3.00 inner diameter .. easy for me to change the cotton..

A set of them is like 5$. Individually it'll cost 3 smackaroos.
SKU 2066300

But if you really insisted to get only one of them, then get the 2.5mm. The red one. 

actually if u want to get any amount of ohm.. it depends on which thickness wire u use too, not only the size of inner diameter..  26awg with 6wrap , u can get around 0.8~0.9 ohm..
yes the subtank screwdriver is 2mm ..

Here, use this. There's an app for smartphones also by the same name ---> Steam Engine

Click the paper clip icon at the top that takes you to the coil calculation tool. Punch in the specifics of your wire, diameter, gauge, target resistance and it will tell you the number of wraps to use.

To answer your question, increasing diameter = increasing wire length used to make a coil = higher resistance. It changes by a tiny amount so for example if I make a 2.5mm dia coil and I get 1.2ohm and one day i feel like making 1.25 or 1.3ohm I don't have to add a wrap, i can just change to 3mm dia coil same wraps.

I usually use 2.5 or 3mm coils so i can sumbat more cotton. it's just a me thing. everyone has their preference, how do you find your preference? Coil everyday and experiment. You will eventually settle on a resistance, diameter, wick style that you like.

im with u bro.. more big diameter, more easy to sumbat cotton.. i just stay with 3mm, maybe will go down to 2.5mm someday .. i love to coil everyday too   .. sure we will learn something

Couldn't say better myself. 2.5mm is more flexible for building.

go get the jig man .. more easy to build a coil..

Anyone else using Singpost when shopping on fasttech? How long does it take to reach here?

Sure. Make sure it says "Authentic. If FT say it's a real deal, then it's real. If not, then it's a clone