Longish post just vanished. Gah! Will try again.
Your situation with your PILs sounds very similar to mine (there's a long thread somewhere in AIBU with all the details).
But in short, after 8 years of marriage, during which time we did have our disagreements and I found them to be quite hard work to get along with at times, our relationship took a turn for the worse when they made some nasty comments about our stillborn son and premature daughter who died and started to tell lies about me and their other DIL to various family members. This started just 3 days after our son died and came to a head when our third baby, now two years old, was about 8 weeks old.
They said something so awful that I couldn't face seeing them anymore and asked DH to quietly explain to them why I was keeping my distance and tell them to just give me the space and peace I needed to come to terms with things and calm down.
Instead they effectively stalked me with phone calls, letters, visits, followed me home from the shops, parked outside the house and stared at it, tricked me into telling them when I would be out alone so MIL could invite herself into the house without having to put up with me.
They spread lies about me to other members of the family and put pressure on DH to make me see them. Every effort I made to slowly rebuild our relationship was met with further bullying, presssure or bad behaviour and just made the situation worse.
It really was one step forward and three steps back, every single time I tried to make things work.
In the end, after they had disowned DH at Christmas because they didn't like the presents we gave them and because we went for tea at SILs house when they were not invited, there was a massive row over the phone, during which they told me that I would do as they told me to do, that I shouldn't have everything my own way, that they had some things I wanted and that I couldn't have them back until I did as I was told and that I was a bitch and they wished DH had never married me.
Their eldest son then got involved, making late night phone calls to abuse DH and finally sending a text that called me a "fucking evil mad bitch who had split up his family by lying about his mum." He also disowned DH.
It really was the final straw and the effects of that argument led to me being diagnosed with panic attacks and grief related depression brought on by stress.
Even DH, who had wanted to resolve things with them, has started to ignore their calls and messages and last week said that he didn't want to speak to his mother as she isn't capable of having a normal or happy conversation and he just can't deal with her misery and guilt trips any more.
I'd hate to think that you were coming to that place with your PILs but I will say that until I stood up to them nobody else in the family had ever done so. When DH and his siblings were small they were all told that if they upset MIL they would be responsible for her mental breakdown. So even though they have had their rows in the past, everyone always does what you have done and apologises to MIL to keep the peace.
So she has spent years being allowed to say and do the most offensive things and wondering why most of her friends just seem to disappear.
I've just finished reading Caitlin Moran's book How To Be A Woman (and I'd really recommend it) and she has some good advice. She is giving an example of how to deal with sexism but I think it would work in your situation.
She says that if a woman starts shouting "sexism" then she is likely to be ignored and accused of not having a sense of humour. But if she says "rudeness" instead of "sexism" people are likely to feel that they have done something wrong and apologise.
So perhaps you could try her line "I'm don't know if you realise but what you just said/did is coming across as very rude" it might work with your MIL.
Or perhaps even "You've mentioned my weight on several occasions now and it is coming across as very rude. I'm sure you don't mean to be so just what is it you are trying to say?"
I hope things work out for you and I hope your DH is supportive of you. Our relationship has been much better since mine realised just how awful his mother's behaviour appears to outsiders who aren't 'used' to it and therefore see it as normal or not that bad.