Depends on which game you’re playing. If you’re asking about AD&D tokens, simply create a token which takes up approximately one quarter of a square to represent a small creature, and create a token which takes up an entire square to represent a large creature. Then, put the two tokens into a rollable table, and follow the normal instructions for creating a multi sided token. If the large creature walks upright like most humanoids, no stretching is required unless it’s a giant. If the large creature is a giant, stretch the token width wise by one square when in this form. If the large creature walks on all fours like a horse, stretch the token lengthwise by one square when taking this form.