THis = This winter is very cold I think we should invest in a bigger heater.
I'm fat. I need to buy Jeans = jeans with a bigger size bigger.

(NOTE: Se Yun, always remember that we write the word "jeans" with small letters. EX: I need to buy a new jeans. / Those jeans look very good.

Next, for the phrase "a size bigger" it is better to write it "a bigger size." EX: I need a bigger size of dress. / He needs to look for a bigger size of the cup. ^^)

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Hi Se Yun! Very good homework. :D Your sentences are getting longer. :) Keep up the good work! I hope that you'll have a lot of fun tomorrow! Enjoy the rest of the night! See you tomorrow! Good night! ^^

~Teacher Lady ^.^~