It's not necessarily about the size of your hands being able to hold it (though a 50mL is unbelievably huge), more about the dexterity of holding it in one hand as your other arm will have the IV in, and then pushing it fast enough, which trying to push that much that fast is going to be met with resistance and could even blow the vein. This is also all under the assumption you can place your own IV. There is also the potential you suffer a stroke midway through and can't complete it, and then survive with a stroke or other nasty complication.