I've done one at 14,000 before, so I know it can go at least that big, but having a million is really of no use.

I've gone as high as 1 million...but that was only for a ship that was large enough to accomodate that amount of units. (Believe me...this ship is huge and it's a gas transport). But, where is the fun in that? Chances are likely by the time you make a few decent runs with it..you've corrupted your character limit of 999,999,999. (Time to start over, eh?)
I try to keep my cargo limits to a reasonable facimilie of what it would be like in real life proportions.
Cargos can be changed to take up a certain volume per unit which would make it more realistic. (Unless you possess an alien technology holographic storage unit)
That's one of the wonderful things about this game. It was done in simplicity..but we can take it to a more realistic level if we desire.

Actually the TCS-Vesuvius has a cargo capacity of 1 million. But you don´t need that much. It´s useless. And it would get to easy to get lots of money.

Entertainingly, if you up the cargo space on an NPC capable ship, such as the Drone, when you see NPC pilots flying it, the space will be upped appropriately.

I am Triaxx, Guardian of Earth.