Here are the items from our mock up:)
I assume your largest white pillows with black tassels are euro size so these are the replacement covers:
Wren Euro - 26"
Basic Sand Euro - 26"
Some ideas for the smallest white pillow replacement :
Summer Tan and White - 18"
Atlas Stripes - 18"
Off-White Thick Weave Pom - 18"
Black/Faux Leather reversible - 18" (might look good and coordinate with your armchairs!)