Your rights at a restaurant when you are ripped off.

Last trip to Thailand I found myself in a war of words with the management of a certain well known restaurant chain in Bangkok.
It was near closing time when I walked in for a late dinner.
The waiter brought me the menu and pointed to the daily special, which was sweet and sour fish.
Realizing it was near closing time, and that much of what was on the menu would not be available; I tapped the picture on the menu to signify that I will take the sweet and sour fish as suggested.
A few minutes later he brings my order, which looks too large for one diner.
I looked at the waiter as I was thinking how to politely phrase my grievance. He didn’t stick around and I am faced with a giant size platter that I will never finish.

Once again I try and attract the attention of my waiter who has made himself busy at other tables.
I eat what I can and I have a fairly good appetite, but I can guarantee that this was a family size platter.
I motion for the bill.
The waiter returns and I notice that I have been charged big money for the dish.
A consultation of the menu details the fact that the fish is sold by the gram. I have no problem with that.
The fact remains that the waiter served solitary me a family platter of fish and this is my issue.

Eventually the waiter returns to my table and I manage to communicate my anger to him.
He points out to me that the policy is that once the dish is brought to the table the customer has to pay. He then promptly walks off as if to say, end of discussion.

My face is getting redder by the second and it's time I spoke to the manager.
The discussion with the manager went something like this.

The serve is family size. There is only me here and if the kitchen has only a family size serve, they should have informed me beforehand.
Any fool can see that one person could not eat that serving.
The manager spun me the same line as the waiter.

At that stage I had planned on getting an apology or some sort of explanation then I would have paid and left, but the manager seemed really aggressive, so I decided to make a stand and demand an explanation.
After a mutual exchange of blank looks, the manager backs down and asks me to pay half.

Victory was mine.
I am not a petty person but my view is that an established restaurant chain in a third world country should be capable of offering professional services to their customers, bearing in mind they add a 10 % charge for service.

What would you have done in my place?