the above suggestions. go with whatever feels right for your space.
in terms of size, two of the most common size pillows are 16 inches and 18 inches. i had two 16 inch square throw pillows on my modern, low back sofa and felt they were too small. that is why i suggest actually starting with an 18 inch throw pillow and mixing it with larger sizes.
try different sizes and number of pillows out in your space and see what you like. 