If you are unable to make it on Wednesday or would like to measure yourself at home this is a great article to read.
http://www.crossfitlondon.ca/wp/wp-content/uploads/2011/03/CrossFit-London-Body-Measurement-Directions.pdf
If you measure and weigh-in at home you don't need to be here until 6:45 on Wednesday:) Just make sure you use the same scale for both weigh-ins and measure carefully.