Fountain pens are what TWSBI are best known for. Unlike most new pen makers, when TWSBI started manufacturing their own range of pens (after years of making pens on behalf of other luxury brands) they went firmly after the enthusiast market. Rather than making simple cartridge-filled pens, they made the sort of quality piston-filler and vacuum-filler pens fountain pen enthusiasts had been missing for many years.

Comes in EF, F, M, B, stub1.1, stub1.5 nib sizes

Roughly the same height as an iPhone when capped.

Slightly longer when cap is posted on the pen. 

Roughly 1 inch shorter than a Diamond 580 fountain pen. 

Please note: Ink is NOT included

A quick no brainer guide in choosing nib sizes in an overly complicated fountain pen world:

If you write small, go with an EF
For the normal writer, F is the most popular
If you tend to lean on a bigger font, M will be perfect
B is for statements!! Sign a check in B, write a header in B, do some colouring/shading in B
Stub nibs are italic in writing style (have a special card or invitation to write?)