The first answer is easy, for resizing creatures there are actually creature models that are called something like "Invisible_Dragon_200" for example. That would give you a critter at 200% the original model size. You then set the model you actually want to use (say, Black Dragon) in the Tail options under Appearance (going off memory here so names may not be exact, but close).
For sitting in chairs, you're basically going to want a script in the OnUse event of the placeable object, that tells the PC to use the sit animation (it's marked as an animation in the script editor, not as an emote). If you type "animation" into the Constants tab of the script editor, it'll show you the different animations to choose from. Then it's a matter of having the script AssignCommand and then it's something like DoAnimation or RunAnimation or PlayAnimation or something like that (can't recall off the top of my head).
For the VFX creature skins, you'll probably want to add a few lines to the OnSpawn script of the creature (choosing Save As rather than overwriting the original or you'll get it on all your critters) and you'll need to both declare the VFX you want to use (for example effect eVis1 = EffectVisualEffect[ -fill in what you need here- ] ) and then on the second line you'll need to tell the creature to apply that affect using ApplyEffectToObject. You'll also want to add in a short delay between the actual spawn time and the effect application (takes a sec for NWN to render the critter) so you'll want to throw a DelayCommand in front of the ApplyEffectToObject.
Now these are very rough explanations, so if this doesn't work for you just fire me off a PM and I can go into more detail
