In the French Army of Napoleon size was not the critical qualifier for being a grenadier - experience and bravery was. Certainly diminutive size would disqualify a soldier from being eligible for the grenadier company of his battalion (but in turn making him eligible for the voltigeur company), but average size was sufficient (and a moustache was de rigeur).
Further, the internal layout of particular battalions was irrelevant for any battle larger than brigade on brigade, which virtually all Napoleonic battles were.
However, the martialing of troops onto the battlefield had long been done by having each unit send guides to mark out the right hand end of its intended deployment. Why the right hand end/side of the unit was chosen was simply traditional, and continued because everyone else did it, and all the drill manuals assumed it. However guides were sent to only one end/side of the unit because if the soldiers had to keep turning their heads to follow two sets of guides, this was certain to disrupt the unit as soldiers swung their heads from side to side and stepped on each other's heels.
Now, given that the guides were on the right hand end/side of the unit, and every soldier's eyes were angled right to follow the guides, where is the most correct and conspicuous place to put your most experienced troops, on whom all other eyes would be set? The right hand side of the unit. Hence the reason for placing the grenadiers, the bravest and most experienced men of the unit, on the right hand side as a model for all the rest of the battalion to emulate; the true place of honour.
The second elite company of the battalion, the voltigeurs, were designated to occupy the left hand end of the unit because they were in practice almost never there. In almost all circumstances they were detached as skirmishers either in front of, or to both flanks of, the battalion.