Finding clothes that fit shouldn't be so hard. Add your measurements here to see which high-street sizes are best for you (how to measure).
How to measure: Measure around the widest part of your bust, the narrowest part of your natural waist, and the widest part of your hips. Wear your usual bra while doing this. Keep the tape taut and parallel to the floor - how tight to pull it is up to you, but you'll generally get the most flattering size results if you pull the tape as tight as you'd like your clothes to fit, in each area!
About: Sizes based on the official size guide published online by each shop - for example, here is the Warehouse size guide. Your closest sizes are then estimated using the least squares method. Skirt and trouser sizes are equivalent in most stores. See the blog post about this project, contact me with feedback, hire me for freelance web development, or follow on Twitter to hear about future projects.