NTRI:
I was looking at either the Mar 22.5 or the 25 to do a short straddle, so below are the probability screens from TOS that I used to determine which strike to pick.
Trading the 25 strike gives me a probability of 52% that the stock will land somewhere in the breakeven range of 18.62 and 31.38. 
Trading the 22.5 gives me a probability of 55.91% that the stock will land anywhere in the range between 16.50 and 28.49.Now I am mostly neutral to bearish on my position, but I have decided to go with the 25 strike since the rest of my trading group is.
Here is the order screen that helps you determine how to size the straddle trade. You take the credit you're receiving and adding the Buying Power Effect (B/P Effect) or margin required, and this becomes your total risk in the beginning. Now, I am sizing straddles conservatively at 1% of my portfolio, but I highly recommend that you do 1 contract until you get used to straddles and the volatility swings.So I get a credit of $6.40 with a risk of $10.00.
