For a designer look, I like to start with either 22” or 24” pillows in the corners of a couch. Most of the time I go with 22” because they will look good on all couches, but go with 24” if you have a larger couch (deep seats, really long, high back).
From there, you layer in pillows getting smaller and smaller in size. If you start with 22” in the corner, layer a 20” in front of it and then either an 18” or a 14x20” lumbar.
For accent chairs, a variety of sizes work just depends on the size of chair. My favorite sizes are 12x24”, 14x20”, or 20x20”.
For one couch, I like to layer 4-5 pillows. For a sectional, I like 8-9 pillows.
For insert sizing, size up at least 2” for a full, fluffy look. For example, stuff at least a 24x24” pillow insert for a 22x22” pillow.
Here is a link to my favorite pillow inserts. They are synthetic down, but hold their shape great and you can still get the “karate chop” look without all the feathers!