Well I've done a bit of quick research this morning with my own collection of organisers.
My new Finsbury (Personal) would take an 8.5 mm diameter pen, or 9 mm at a push. Where as my used (2-3 years) A5 and Pocket size organisers would both easily take a 10.5 mm diameter pen. I used the shank of a selection of twist drills to measure the size each would take accurately.
In terms of actual pens I'm using Parker pens in my larger organisers and a Filofax 'Barley Twist' Mini pen in my Pocket size Filofax, most other pens are too long. On my A5 I carry a Parker fountain pen and a ball pen.
The Filofax pen is quite heavy for it's size, it has a retractable ball pen refill. One tip, always retract the point on this pen, if you drop it as I did once the pen is so heavy that it will damage the point if it lands on the point as mine did! I did buy a pack of refills from Filofax and the refill is very slim compared to the size of the pen itself!
What pens are you using?