We have the following expression: 
 ________ big as
 For this case what we should do is find a word to start a sentence that is for comparison.
 The correct word in Spanish is: 
 "Tan"
 The sentence in Spanish is:
 Tan grande como
 And the translation of the sentence in English is: 
 As big as 
 Answer:
 the proper word to begin this phrase is:
 "Tan"
 Tan grande como