The result will be a small living boy weighing approximately 8 oz. or one half a pound - or, alternately, for metric sizes, 250 grams (throughout this post, metric weights will be slightly more than imperial weights). This boy will be correct proportionately and approximately 8 inches in height. In character the boy will be malicious when fed, ravenously dangerous when hungry.
On its first day of life, the boy's appetite will crave 1 lb. of living matter (or 500 grams), half of which must comprise flesh, bone, hair, fur or hide, even tanned hide. This will be gobbled quickly and the gingerbread boy's weight will increase by half the weight of the food, or 8 oz. (240 grams).
Each day thereafter the amount of food that the boy will require will double - 2 lbs., 4 lbs, 8 lbs. (500g, 1kg, 2kg, 4kg) and so on. Each day, the gingerbread boy's size will grow proportionally, increasing by half the amount of living material eaten. The boy will quite willingly eat anything that's alive, up to half its own weight - so that when the boy reaches 8 lbs. (4kg) it will be large enough to eat rats, at 16 lbs. (8kg) cats and at 32 lbs. (16kg) small dogs and so on. The boy will double in size until it is as large as 4,000 lbs. (2000kg), whereupon it will collapse from its own weight.
Feeding time must be the first hour after the sun rises; if the boy is not fed by the end of that hour, it will become infuriated, making noise and attempting to break free from whatever confinement it has been placed, squeezing through bars or easily climbing up the smoothest of walls, seeking food on its own. Once fed, it will become passive, though it will act out abusively towards others, causing animals in particular a point of damage each day if its creator is not there to watch it and sharply reprimand its behaviour (more or less continuously until the sunset). If the creature does not receive its food by sunset, it will die.
While the boy, when passive, will do what it's told by its creator, this will only last until the creator becomes small enough to eat (that is, when the creator's weight becomes less than half of the gingerbread boy). At that point, the boy will begin to do as it likes, which will mean randomly attacking anything or anyone within reach.
The boy will possess 1-8 hit points per 50 lbs. of weight, but will only attack as 1 hit DIE per 250 lbs. of weight. Thus, the boy will have 10d8 hp when it has reached 512 lbs, but will only attack as a 2 hit die monster.
It will do a maximum of 1 damage per 25 lb. of weight, divided between two attacks (the right hand will be the stronger). Thus, at 512 lb., the boy would do 1d10 damage with each fist. At 1,024 lbs., this would be 2d10 with each fist.
While still under control, the gingerbread boy will gleefully enter combat, attacking enemies of the creator (eating them if small enough and the boy is still hungry), so that the boy will be useful until it reaches a size too large to be controlled. A wise player will prepare in advance for when it is time for the gingerbread boy to be killed before it becomes dangerous.
If the boy senses that it is abandoned, it will 'know' the location of its creator and will seek to reunite if able. The boy will only prefer to eat its creator if no other creature is available - such as the creator's party, animals, enemies, children in particular, etc.
Keep in mind that creating the gingerbread boy and releasing it at some point in a town (for example) will be something that draws attention. The divination spell, cast by a 7th level cleric, can very easily determine the source of this sort of thing with two questions: who created the creature? and where is the creator now?
Food for Thought.