One of the special awards available in the Slytherin Quest is the Size Matters award. The rules state that for size matters you must "complete a regular class homework that uses at least 600 yards. This award does not apply to Detention projects, any Advanced Studies or Quidditch projects this term."
So, here are my suggestions for size matters this month. We still have 20 days left, which should be ample time to craft projects of 600 yards or over.
Flying would be the perfect class for this, with the option to craft pants. If there's a little lady in your life who would appreciate them, I suggest Emma's Unmentionables. You customize the fit as you go, so do make sure to weigh them and make sure you've used 600 yards, but they use a DK weight yarn and make a totally cute pattern.
You can do them in a solid pale colour like this example, or there are stripy, textured, patterned versions in the projects. These might be great for using up a bunch of half balls of DK weight.
Herbology's option three: something pleasant to improve a forgotten area just cries out for soft furnishings. Blankets, cushions even a lampshade could be really great for this. I suggest a cushion. Again, you'll want to weigh your cushion cover to make sure you've used 600 yards, but you could pick a crochet stitch or motif and simply make a cushion cover from two squares of the right size. Lucy of Attic24 has a fabulous tutorial for backing a cushion with a thrifted sweater, but you might need to crochet both sides of the cushion to reach your goal of 600 yards. If you need a pattern to follow, I suggest the Copenhagen Pillow. It uses 600-800 yards of Aran weight yarn but again, could be a great scrap-buster for the rounds of the granny squares.
Option one in Charms asks for a bag in which to practice your undetectable extension charm. There are heaps of great bags out there, and I had great fun looking through them. The one I've chosen to recommend is the Rainbow Hexagon Beach Bag - what can I say, I love rainbows. It uses worsted weight yarn, and, being a beach bag, would also work very well for Potions this month. Pure cotton is a pretty summery fibre, after all.
Finally, for Care of Magical Creatures, I suggest knitting a shawl. Something like scrample you can just keep on adding to until the deadline. It was designed to use scraps, samples and leftovers, and will work with any weight of yarn.
Done in a dark colour or a grey to hide in the moonlight it would fit under option 1: blend into your surroundings. Done in another colour, it would be a comfortable object for a mooncalf to have in their burrow, for option two.
I hope that you get some ideas from this as to how you can fit size matters into classes this month.
Enable one another.