I would have to agree with thaxos.. 3mm will work just fine (have have made a couple sets of armor from 3mm).. with a single 4'x8' sheet you will have more then enough for your armor and possibly a second set of armor as well.. One piece of advice though for the gauntlets and helmet ( if your gonna make them as well) use 1mm as it is easier to get to form for a tighter curve