Oh! I'm so sorry I forgot about that! Chukkle was right in that a Raiment is an ideal weapon for the individual knight, so yes a set is fine. There are no size restrictions per se, a Raiment that is a siege weapon has never before been projected however weapons with similar capabilities have. For instance a magic bow that fires astral arrows with a great many properties, one of which being the ability to explode on impact. 

And no you cannot just summon your Raiment out of thin air, you manifest it and then it's yours to drag around forever. However there is an incredibly advanced Magistra that has that effect. Though as far as I know the only people who have mastered said Magistra are on the Council.