Changing player heads size that are on the ground

Get your resource pack and go into resource pack/assets/minecraft/models/item. Now create a file called "player_head.json" (make sure it is .json and not .txt) and paste this in:

{
    "parent": "item/template_skull",
    "display": {
        "thirdperson_righthand": {
            "rotation": [ 75, 125, 0 ],
            "translation": [ -1, 3, 0 ],
            "scale": [ 0.8, 0.8, 0.8 ]
        },
        "ground": {
            "rotation": [ 0, 0, 0 ],
            "translation": [ 0, 3, 0 ],
            "scale": [ 0.5, 0.5, 0.5 ]
        },
        "fixed": {
            "rotation": [ 0, 0, 0 ],
            "translation": [ 0, 0, 0 ],
            "scale": [ 1, 1, 1 ]
        },
        "firstperson_righthand": {
            "rotation": [ 0, 135, 0 ],
            "translation": [ 3, 1, 0 ],
            "scale": [ 0.8, 0.8, 0.8 ]
        },
        "gui": {
            "rotation": [ 35, -145, 0 ],
            "translation": [ 0, 1.25, 0 ],
            "scale": [ 0.8, 0.8, 0.8 ]
        }
    }
}

Now you see where is says scale in the ground section? the default is 0.5 but you can increase it to increase the head size. I suggest using 1.