Yeah this. ^
I'm so sorry @WildflowerGardens there is no answer to this apart from moving out and getting your own place and giving her a wide berth, or sticking with it. My mum was pretty much the same. Very critical of me (and others sometimes, but mostly me- being the only daughter. My brother could do no wrong though!) 
My brother never lifted a finger to wash a cup, but he was the blue eyed golden boy. I (unfortunately) did just tolerate it, even though she could be cruel with her barbed comments. I had no choice really, unless I cut her off, which I did do a couple of times, but it lasted just 1-2 weeks, as I felt so guilty, especially as various extended family members made me feel like shit for it, and said I was a 'disgrace.'
She often picked on my weight and clothes size, especially when I was at my 'largest.' I was a size 12 to 14... (In todays sizings - it's maybe a size 10!) And she picked on my driving, my job, my parenting/mothering skills, my choice of decor in my home, etc etc..... I could never do a thing right.
She was argumentative and rude. Not ALL the time, but maybe 35-40% of the time I was with her.. That was enough of course. I 'coped' with it by changing the subject, and starting to chat about something else. Last night's Corrie, one of her neighbours or one of mine, or a family member, or one of my friends, or one of my DC, or something in the news, anything. Then she would leave off me and start talking about something else. Almost like aversion... Like you do with children when they misbehave LOL!
I did call her out on her spitefulness towards me, quite a few times, but she went into a huff, and sulked, and got quite shirty and nasty with me, saying 'I'll just seal my mouth shut with cement then shall I and never speak!' And she would act like the wounded one, and then she would still carry on as before the next time I saw her, as if the conversation hadn't happened, and would be insulting me again!
If you are not able to stop seeing your mum, (or see her much less) maybe try something like this? If you have tried, and aversion isn't working, sadly, there's no answer other than to try and leave, and see her much less. There really is no other option. You can't change people. She is being very cruel and nasty, and you deserve better.
But no you're not too fat for a relationship! This awful scenario with your mother will be the main reason you're still single... It's sucking the life - and the energy out of you. Good luck. At 5 ft 4ish, I have been between 8 and a half stone and 15 and a half stone, during the 35 years with DH, and he has loved me and fancied me no matter what size I am. I am currently somewhere in the middle of that!
Sorry you're going through this. 
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