" 1. you will break several law of physic
2. you probably get more milage building a huge state of the art tank filled with guns
3.Lighttsaber is inherently MORE impratical ( you will still die in one shot to the head) it is close combat weaponry , not much use now , useful in medieval but not now
4.if we break several law of physic, say we invented magic or somekind of new reality breaking technology, a medium sized mech would be the best, not taller than a 3 story house , rather than the giant hulking machine we see in say gundam or EVA(definitely not EVA), see gasaraki and flag for military application
this video shows a more realistic outlook on the use of mecha or play cromehounds "
if you've got a little bit of military otaku in ya.