I am an absolute beginner in fountain pens, however my interest has been extremely peaked.
I currently carry a mini ballpoint pen in my front pocket. I would also like to carry a fountain pen like this as well. I have medium size hands with medium writing. I would like to move to bottle ink in the future but this is not a must in the beginning.
There are several pocketable fountain pen options but there are some trade-offs when getting a fountain pen small enough to fit in a shirt pocket. Some will be shorter overall, even posted, than a regular full-sized fountain pen making them less comfortable for a longer writing session but acceptable for note-taking. Some will not accommodate ink converters because of the smaller size and will need to be used with cartridges only. You can refill cartridges with a syringe though so there is a work-around for this issue.
The first brand to come to mind is Kaweco which makes the Sport line in plastic, aluminum and brass (even carbon fiber!). There are many nib options available from EF to BB and prices for the Sport line start around $25 for a plastic model and go up to $100 for the brass models. Pocket clips can be added. For larger hands, the Raw AL or Brass models might be the most comfortable because of the added weight. The plastic models are quite lightweight but I find adding the clip and posting the cap help balance the pen.
Kaweco also makes a smaller pen called the Liliput which is quite pocketable but is not as comfortable in larger hands for writing over longer sessions. But is comparable in size to the Fisher Space Pen.
On the budget end of the spectrum, there is the Pilot Petit Mini Fountain Pen which comes in eight translucent colors with matching inks, each for $3.80 and features a fine Japanese nib. If you wanted to dip your toe into the pocket fountain pen world, this is the least expensive way to try it out. The Pilot Petit is refillable, three cartridges are $1.90.
Another great small option would be the TWSBI Mini ($50) and it is a piston filler, designed to use only bottled inks. It’s small enough to fit in a shirt pocket but the cap will post and thread to the end of the pen to give a comfortable writing length. TWSBi uses European nib sizing and the nibs come in EF, F, M, B, stub1.1, stub1.5 nib sizes so there are plenty of options to choose from. There is also the newer TWSBI Vac Mini which uses a vacuum filling mechanism which might be a bit trickier for a new fountain pen user but may be something to consider later.
The Taccia Covenant is a higher end pocket fountain pen that lengthens by hiding the body of the pen inside a cap of equal length. When posted, the pen is over 7.25″ long. It’s available in three colors with a two-tone steel nib in fine, medium or broad for $103.20.
Franklin-Christoph has made a couple pocket fountain pens and I particularly like the Pocket 66 in Ice, eyedroppered. By sealing the threads with silicone grease, the whole barrel can be filled with ink both showing off the color and maximizing ink capacity. Prices start at $149.
I hope this gets you started on your quest for the perfect pocket fountain pen. The pocket fountain pens are some of my favorites to collect and I’m sure you’ll enjoy adding some to your collection too.