For a UK standard brick
you will need to knit:
i) two ends at 4" x 2.5"
ii) two sides at 8.5" x 2.5"
iii) a top and a base at 8.5" x 4"
I used the standard tension given on the yarn ball band to calculate that my pieces should be
i) 22 stitches, 18 rows
ii) 46 stitches, 18 rows
iii) 46 stitches, 28 rows
I knitted the base in garter stitch (all rows knit) and all the other sections in stocking stitch (alternate knit and purl rows). The pieces I made out of stocking stitch were rather curled so I pinned them to my ironing board and steamed them to flatten them out a bit. If, like me, you have used acrylic yarn do NOT touch the knit pieces with the iron as they will melt.
My sides were a bit long, but this excess was largely taken up by the bulk of the wadding, if I made this again I would only cast on 42 stitches to give a snugger fit, BUT this does depend on your personal tension AND the size of your brick.