Kids learn about size in context and the more you work with them with examples, the more they understand. Here's a simple way to work with your kids on sorting by size. These types of simple parent resources are terrific and many momstown chapters have a parent 'make and take' night to create these tools to take home and work with your child.
We created several school related items using felt for our felt board (see our blog here for creating your own felt board). We practiced sorting the different choices. You could use this for any theme and you could create a variety of objects. We used buses, apples, crayons, and chalkboards for "back to school".
All you need is a variety of felt, scissors and a theme (school, shapes… etc).