Sure, here are the correct forms of the words in parentheses:
I think you need a bigger size. It is really small for you.
What about this red one? It is even better.
Look, it is on a special offer. It's cheaper than the blue dress.
The blue one is from the new collection. It is more expensive.
Here is the corrected sentence:
I think you need a bigger size. It is really small for you. What about this red one? It is even better. Look, it is on a special offer. It's cheaper than the blue dress. The blue one is from the new collection. It is more expensive.
I hope this is helpful!