I definitely do a lot more playing with weapons than armour (though I've made lot's of armour sets for myself). What you do to edit armour (or anything really) in Nifskope (for sizes) is to right click on the object (it should be hi-lighted with white lines everywhere) and click "Transform" to edit where it is on the axis (I wouldn't recommend doing this with weapon hilts/handles, since it messed up where your character holds it).
Furthermore for Nifskope, (I'm not sure if they updated or if my version is old) but if you press "view" you can look at a "block list" which will allow you to edit textures they use, to edit how the texture looks on the armour you can right click the armour again and hover over "Texture" and press "UV Edit."
My tip for you is to just play with Nifskope. Don't worry about whether or not the mesh will work in the construction set, just play with it for a little. Making meshes for fun is a great way to learn how Nifskope works, it's a bit weirder with creatures, but if you're making furniture just about any mesh you do will work. Just make sure that when you combine things in Nifskope you give them a "parent-node."
The best thing is you can learn through trial & error, what works and what doesn't work. It may seem like a slow process but it helps very much when making meshes, and the first thing I learned in Nifskope was editing texture paths through doing this, and making many meshes where some worked in-game, other's did not. But I learned a lot on how to make my own armour, weapons, architecture tiles, et cetera.
I also have some pictures if you don't quite understand what I'm talking about or simply want visuals (I know I like visual pictures when it comes to .nif editing.
http://s30.postimg.org/rezs8rfoh/Nif_Armour.jpg (Resizing/Moving/Rotating)
http://s29.postimg.org/pccbxrohj/Nif_Armour2.jpg (Block List/Block Details)
http://s11.postimg.org/hbz058r6b/Nif_Armour5.jpg (Texture Editing)
http://s10.postimg.org/axkjzn555/Nif_Armour6.jpg (Colour Editing)
http://s15.postimg.org/bmlajcyfv/Nif_Armour3.jpg (UV mapping)
http://s1.postimg.org/5es4omtdb/Nif_Armour4.jpg (Nodes)
http://s21.postimg.org/3v0w3uqs7/Nif_Armour7.jpg (Copying)
http://s30.postimg.org/rasn6nmzl/Nif_Armour8.jpg (Pasting)
When pasting something to armour, make sure to change the newly pasted object's "ID." In this example, I copied "Tri Chest 0" and pasted a new "Tri Chest 0." This isn't game breaking to have two "Tri Chest 0's" but it will mean either both "Tri Chest 0's" or all "Tri Chest 0's" won't appear when you put on the armour, so name the new pasted objected "Tri Chest 8" like in the example, since you have to do the next number in line that is not taken. (So for this Bonemold armour mesh, I have to do 8 since numbers 0-7 are taken).
Furthermore, it's best to take things from other armour and clothing if you want to make a new outfit, rather than, say, static items as those have not been made to be worn! Finally, ideally your armour "Block List (on the left)" should look something like this if you want it to work as you intend it in-game:
http://s7.postimg.org/rbkmoaxrv/Nif_Armour9.jpg