This is a simple circle generator.

How to use:

1. Choose the block by replacing the brick block.
2. Choose a size by going o the second repeating command block (the one with the clone command) and change the value next to the ^ it's initially set as 8.
3. Get a name tag and call it CircleGen.
4. Place an armour stand where you want the centre of the circle.
5. Use the name tag on the armour stand

When doing large circles there may be a few blocks missing.

If you need help leave a comment.

Here are the commands if you want to build it yourself:

Repeating Command block 1:
execute @e [​name=CircleGen] ~ ~ ~ tp ~ ~ ~ ~.5

Repeating command block 2:
execute @e [​name=CircleGen] ~~~ clone -7 4 2 -7 4 2 ^ ^ ^8

Impulse command block (720 tick delay):
kill @e[​name=CircleGen]

See the images for the layout.
Change -7 4 2 to wherever your block of choice is.