Answer:
Here are some good ones that I like:
Your mums so fat, she once wore a Yellow t-shirt and someone said "Taxi"
Your mums so small, she committed suicide on a kerb.
Your mums so dumb, she was locked in Tesco and starved to death.
Your mums so fat, It took 2 hours to drive round her, and I ran out of petrol.
Your mums so fat, she has her own postcode.
Your mums like a London Bus, pay £1 and climb to the top of her.
If you want more, I recommend Googling it.