I saw a boulder.it is a very big size and smooth.but big boulder next to the rock is very small.

Eric, 

Thanks for doing your homework. =)

-Teacher Ash

I saw a boulder.

>> CORRECT!

it is a very big size and smooth.

>> It is very big and has a smooth texture. 

but big boulder next to the rock is very small.

>> However, the rock next to the boulder is very small.