One factor that may cause an increase/decrease of the response rate is the length of the email. Actually, there is no guarantee that a lengthy email yields higher response rate than a shorter one. The number of words that must be used would mostly depend on the type of outreach you're doing. For product reviews, you might write a longer email since you need to explain the unique proposition of your product and use more words to entice your blogger to write a review about it. While if you're just sending an email to a person to let him see/share your content, you just need to be direct to the point and use 3 to 5 sentences to tell him your offer (since end-receivers in those kind of outreach don't want to read fluffy words in their inbox).

The main point is that you shouldn't care too much about the number of words that you will use in your emails. The more concise your email yet should be understandable, the better.