Is this international size cartridge?

waterman makes one size fountain pen.. it doesn't make alternate cartridges for overseas... Its a waterman cartridge only.

Will this short international ink cartridge fir my nemosine pen?

Hi, this is Ivana from Waterman Consumer Care. Thank you for contacting us regarding our Waterman Fountain Pen Ink Cartridges. Please be advised that we have never our cartridges on other brands' pens. Our apologies.

I also received the mini ones, didn't realize it. Now I don't have a pen that fits it. What pens will it fit?

The pen that I have which these cartridges fit is the Waterman Lady Agatha. However, I believe that they will also fit pens that take the "universal" cartridges, although the Lady Agatha won't take them. Hope this helps!

Will this cartridge fit the Muji aluminum pen?

Thank you for your interest in our Waterman brand. Sadly we do not test our products with other brand, you will have to check it with the producer of the pen. Our refills are international style of refill, so if Muji is, it should work just fine.