Well, you could argue that the goods supplied were not as described, and get your money back, but to be honest, that's a fine looking plant, whatever size pot it came in, so its up to you really. If you paid a premium price because it was in a 10 litre pot, then argue the case - it certainly looks as if it will need a bigger pot fairly soon though.

I would check if there is a price difference between 5 and 10 litre pots and ask for the difference from them. I would keep the plant though as it is a good size and looks attractive. Judging by other Phormiums I have seen in GCs the one supplied is large.