See the questions a bit so I'm going to cover them quick since I'm doing the process myself, here are two common problems with exporting in Blender I see people have and how to solve them with ease.
First is exporting an object being a weighted object and retaining the size and position. The idea is to add the position information from the bones to the obj model. This is as simple as highlighting everything. For future reference always highlight everything you export be it a 3d model, a nif or otherwise. What that does is select "all" and tell Blender it is exporting everything. This makes certain it isn't missing anything. Select all is simply A on the keyboard. Everything will be highlighted with a pink glow. Understand though that if a static object is the target it may not retain that position. It isn't assigned to the Scene hierarchy in the first place and rather has it's own position. In Nifskope you can see this and it is set there in the program to relative which tells the game engine to place it at a relative position from it's own axis point to whatever part of the hierarchy or scene it has been attached to.
For exporting on obj I'm using the standard male basic effects for an example. Right click on the model. Then Shift + Right click on the armature as shown.
(http://catman.freedomforceforever.com/temp/exportA1.jpg)
Click Ctrl + I to invert the selection and hit Delete to get of everything but the model and armature.
(http://catman.freedomforceforever.com/temp/exportA2.jpg)
Click A to highlight everything left over which is the model and the armature and export the obj. When you open the obj in another program the armature will not be there however again if you open the obj in a text program you'll see all it's position coordinates have been retained. Exporting on obj without the armature otherwise leaves it without the positions in the nif scene.
(http://catman.freedomforceforever.com/temp/exportA3.jpg)
Second is the non-uniform error. This happens when weighting a model. The reason in short is there's extra information in there. In my example I'm using Rens Vigilante and there's a static node that he added and otherwise still in there that needs to go because it doesn't pertain to the model I'm weighting, it isn't part of the native hierarchy. Taking it as read but to be certain the model has been weighted to the original which was male basic and this was done using the Bone Weight Copy script with an X-crossing factor of 2. After the old male basic was deleted and the new model made parent to the armature by right clicking the new model, a wig in my example, shift + right clicking the armature (skeleton) and clicking Ctrl + P. I then export the nif but the error prompt comes up. To resolve this click A to deselect all then A again to select all then Ctrl + A. This brings up a prompt, select the first option which is Scale and Rotation to ObjData. You'll notice the bounding boxes change a little after that and that means you're good to go now. You'll be able to export just fine after this.
(http://catman.freedomforceforever.com/temp/exportB1.jpg)