The potion of growth and potion of shrinking can be crafted in a brewing stand using leather and thick potions or rabbit hide and mundane potions respectively.

If you drink a size potion it will shrink/grow you past whatever size you currently are; drinking a growth potion while tiny makes you normal sized, as does drinking a shrinking potion while giant.
However, if you drink a shrinking potion while tiny then you will instantly die, the same applies to drinking a growth potion while giant.

0.25 times scale, tinies have most of the same stats as normal players except for the fact that you have 5 seconds less oxygen, and have 2.5 hearts, which is made even worse by the fact that you are now a target for smaller predators such as foxes and cats.
However being tiny also means you can fit in small spaces, and survive a fall from up to 25 or so blocks due to your lower terminal velocity. You can also sit on normal sized player's shoulders.

2 times scale, you are noticeably faster, have to eat much larger quantities of food, are immune to knockback and are much stronger with 5 melee damage and 30 hearts.

Being huge means some mobs that are usually hostile will not attack unprovoked, but you are also feared by many mobs including villagers, and iron golems might also see you as a threat (they can still annihilate you btw). You are also more vulnerable to fall damage than smaller players, a fall that would normally cause minor damage could be deadly to a giant. Normal sized players can also sit on your shoulder.