Jun: the state is small

correct: The state is small.

Jun: our flags color is red,whit,bule and blak

correct: Our flag's color are red, white, blue and black.

Explanation: Flag is a possessive noun in your sentence. A possessive noun is a noun indicating ownership or possession. Use is if singular, and are for plural.    

Jun: the world map is big

correct: The world map is big.

Jun: we have a big statue

correct: We have a big statue.

Jun: we have a plain ground

correct: We have a plain ground.

Explanation: Your sentences are correct there is the subject and predicate. But be careful Jun when you are writing a sentence, the first letter of your sentence should begin with capital letter and ends with period.  And also check your spelling. Good job Jun! See you can do it. :)  

Thank You so much Jun. See you in our class ^^