Usually it makes the most sense to look at what you have to work with, ie the dimension of your existing room, the size of your bed, etc. to see if you have enough space to add anything extra such as the chaise lounge shown, or to help determine the size of area rug that will work best. We usually do floor plans for our clients to help plan the space before new new items are purchased. It's definitely better to start with a design plan for the space you want to decorate.

PS... Here's a blog post I wrote on design plans...
http://devinedecoratingresults.com/2010/01/16/choosing-your-decorating-style/