OK so I’ve got the knitting needles out and had a good rummage through my stash so I can give you a couple of ideas for 8″/20cm squares.

Stocking Stitch Square
Using 6.5mm needles and standard chunky yarn cast on 25stitches loosely. Working in stocking stitch (1 row plain, 1 row purl) knit until your piece of knitting is square ending with a wrong side row.
Cast off loosely.
Hint The best way to check this is to fold the bottom right corner over to the top left corner diagonally. If the edges meet on the left side your piece is square.

Moss Stitch Square
Using 6.5mm and standard chunky yarn cast on 24 stitches loosely.
1st Row: Knit 1, Purl 1 to end of row
2nd Row: Purl 1, Knit 1 to end of row.
Repeat these 2 rows until knitting is square (as above). Mine took 44 rows.
If you’re making squares remember it’s easier to block slightly smaller ones to 8″ but we can’t make larger ones smaller. Having said that we will use them. You can also use 2 strands of double knit yarn held together to make a chunky yarn.
Happy Knitting!