Smart Goal Assessment Using Word Count

Mindee asked:

Could someone help me with the idea of assessing via word count. I’m going to pretest w/a series of pictures (story) and ask kids to write as much as they can about the picture. I’ll do it again at the end of the year. How do I know what is a reasonable amount of words (sentences?) for a 4th grade class to be able to write? Would I be better giving them questions/true false/cloze type questions?