The cards are scaled to be printed out on A4 paper, but I actually ended up printing them out of B5 paper.
I also recommend backing the cards with cardstock, so they are more durable and so that the design cannot be seen through the back of the card.
The weather is Sunny, Cloudy, Rainy, and Snowy. The numbers are 1 though 20. It is designed to make 2 total decks. Just print the "1 to 9" and "11 to 19" sheets twice each, and print the "10" and "20" sheets once each. Doing this will also give you an extra Snowy 10 and Snowy 20 card that you can discard. You can print more or less as needed.
Once you have these printed out and ready, you can play any applicable card game with them. For larger class sizes, I recommend Karuta, since it only requires you to print out as many decks as there are groups. For smaller class sizes, you can play Go Fish, which will also let you practice "Do you have ~?", "Yes, I do", and "No, I don't".
I'm also including my original GIMP/Photoshop file, just incase anyone wants to change this template.