Place two objects on the table one being larger than the other, they can be similar objects or completely different objects for example use a small toy block and a large toy boat. Ask the child which one is bigger, ask the child which one is smaller.
Another way to go with this activity is to provide the child with lot of objects which are big and small and have them sort the objects into buckets, a big bucket for the bigger items and a smaller bucket for the smaller items.
Use your themes and make cards with pictures of bigger and smaller items, or animals on them. If your theme is transportation you could make cards with pictures of real cars, trucks, airplaines ect...and cards with pictures of similar toy versions of the same items being played with by children...see if they can differential between real and play items, the real items are bigger in real life, the toy items are smaller in real life.
Make file folder games using your theme, if your theme is snowmen or a wintery theme of some kind you could make two different sized snowmen colorfully dressed and have the children sort bigger and smaller snowmen and talk about the differences in size as they play the game.
All of these suggestions are also good small motor activities.
Get creative and have fun teaching the concept of size differentiation.