Animal eyes/fish eyes
The "eye" of a flower
Spots on a faun/leopard
Dew drops on a leaf, beads of moisture
Rain drops on the sidewalk
Spider's egg sacks
Beads of sweat
Ripples on water from a dropped pebble
Dandelion seed head
Seedless watermelon, cantaloupe, honeydew melon
Basketball, baseballs, golf balls, tennis ball, ping pong etc.
M&M's or Smarties
Zeros and the letter 'O'
Water bottle lids
Sphere, globe, circular, ring, round, loop, halo, band, loop, orb, orbital, ball, pellet, disc
Describing a shape is best done in as few words as possible. Think of the shape as a camera snap shot--you want to capture the gist of what you mean as soon as possible so you can get on with other related (and more important) detail, and the action happening in the scene
A weak example:
The zit on his chin was as greasy and round as a slice of salami.
What's wrong with this example?
This description's pretty good nasty-wise, but the size of the comparison is all off. We get the image of a massive zit so comical that it might spoil the description unless humor is intended.
A strong example:
The last thing I expected at the wake was to see my estranged cousin Sarah working the crowd, playing the part of the heartbroken granddaughter. When I noticed the twin jade globes dangling from her ears, catching the light, my hand twitched with the urge to slap her. Not only had she found Nana's spare key, she'd gotten into her antique jewelry collection as well.
Why does this work?
This works because the shape and size is accurate enough to give the reader an image, but doesn't detract from the emotion tension of the scene.