(in photo two 24" squares, one 22" square and one 18 x 26")
How do you know what size throw pillows will work for you"  Liana, "The Pillow Master" has a great eye and is an excellent seamstress for all soft fabric décor.   Here are her sizing tips for pillows.

(Incidentally, Liana is beautiful and sort of shy...as in "doesn't like to be in the limelight"...but I dragged her here anyway :)

The rest of this post is thanks to Liana...

An interior designer will be able to determine the right size and number of pillows to best suit your furniture.  And as I normally tell my clients:..." hiring a designer, is more affordable than you might imagine, and not only will save you time, and headaches, but also will keep you from making expensive mistakes."But, if you enjoy tackling the challenge, here are some tips for selecting adequate pillow sizes for sofas, chairs, and banquettes. Note : These are not rules, but simple "fool proof" tips to create seating arrangements that are comfortable, balanced and in good proportion.
A pillow cover, when laying flat, will have a certain height and width (in this case we have a 20" high x 20" wide pillow cover). 
 Once stuffed, the cover will gain another dimension, and its depth will be a number of inches taken from the height and the width of the flat cover. 
In other words, once stuffed, your 20"pillow cover will no longer m...