This mod provides a config which lets you change the scale, attack strength, speed, jump height, and such of nearly any living entity in the game.
The mod will generate a full list of defaulted config values. This mod won't change anything by default, until you change the config.
minecraft { player { D:armor=0.0 D:damage=0.0625 D:health=0.5 D:jump=0.5 D:reach=0.0625 D:scale=0.0625 D:speed=0.25 D:toughness=0.0 } }
As an example, this config will put the player to the size of chisels-and-bits, including their jump height and speed.