I like to make patchwork blankets too. Here is what I do:
I usually make mine with sportweight yarns and knit them on #5 US needles. I usually cast-on about 20 - 25 stitches and then knit about 35-40 rows for each patch.
There is an easy trick to make sure your patches end up as perfect squares -- fold each patch and turn it into two triangles. When they are the same size you know you have a perfect square.
Try this while you are knitting: With the patch in front of you and your knitting needle on top - take the bottom left corner of the patch and bring it up to the top right corner. When the triangle on top is the same size as the one on the bottom then you know you have a perfect square. If you can't get the bottom left corner to touch the top right corner then you know you need to keep adding more rows.
Then I sew all the patches together and finish them off with three or four rows of single crochet. I attached a pic of one of my finished blankets.
Hope this helps