I plan to take the Ghostly Warder virtue but am a little unclear on how you generate the Magic Might total and how to determine what Powers it will have etc
Can someone point me in the right direction?
Also, I'd like to give it the Magic Sensitivity ability, is that acceptable? If so, any suggestions on what speciality would fit?
Noble's Parma: I believe there's an exemple of Ghostly Warder in the corebook.
And yes, you can have magic sensitivity, with or without a warder.
Fixer, He's asking whether the Warder itself can have Magic Sensitivity, not his character.
Yeah that's just an example. I want to know HOW you determine Magic Might and what Powers it can have. The example has Kinesis as a power, which is nice, but I'd like to know what other powers they can have or is it simply "whatever you want"
Sorry, I was doing way too many things at once 
Magic Might should be roughly equal to the given might, with powers linked to a theme. I'd say at most 15 (5 per virtue point).
If the warder was a fire magus, he may have fire powers just as the ghosts in Calebais. Most "ordinary" ghosts, however, will probably have mentem-based powers.
Cost of powers? Magnitude/5 is a good guideline. And I'd about give 1 power/10 might, or 2*Might levels in powers, something like that.
However, this is just my point of view. Mostly, if your troupe and SG agrees, then it's ok
Considering that WE will be his troupe, I'd say thats a good start for discussion. 
If you put it this way, sure
wanna sleep, wanna sleep...
Ok, you lost me. What's the "given might"? How is Magnitude determined?
Sorry for appearing stupid but the rules don't seem to state HOW to go about creating a magical creature (which this effectively is)
I dont beleive this is correct...Magic might is equal to the might.
One is the number of points to use on powering effects the other is...it's magic resistance. DOE would lower the deamons "magic resistance" might and not the points available. The might points replenish with rest, other heals like wounds.
Confused yet? it doesn't matter what you call them as long as you keep them separate.
Magnituded is level/5 so a level 15 effect is 3rd magnituded.
decide what you want it to do, decide how powerful it should be to do that, give it some powers that fit the idea you have. in general a might of 5 is like a goblin/kobald and might 50 is a deamon lord. A ghostly warder is probally 15ish.
Excellent suggestions Agnar.
Gino, another way to look at this conceptually I would think, would be to ask yourself "How accomplished was my Ghostly Warder in his field/profession in life?", "what was it that made him stick around on the material plane after death?". Was it a curse upon him, and if so then how powerful a curse? Was it through some magical agency like Summoning that brought him back and attached him to you?
These could help to give you a sense of what sort of Might you should give him (within reason of course) and the sorts of powers you might ascribe to his spirit nature.
I of course could be wholly incorrect and would equally welcome thoughts from many of the more experienced players/SGs/line writers(especially) on how they view creature creation concepts and mechanics.
Sorry, it seems you both misunderstood me, as I expressed myself quite badly.
I meant that the magic might for a warder should be roughly equal to the might given to the exemple warder in the corebook.
Well, ok, here's a brief outline. The Ghostly Warder is the magus's father who deserted him before childbirth and has been racked with guilt ever since. He went on to become an accomplished diplomat/spy in the service of the Spanish government. He planned to retire and reconcile with his son but was killed whilst "on mission", so to speak.
It's this unreconciled guilt that has him tied to the material plane. He has vowed to protect his son as much as he can. I figure a Might of 10 or 15 would be suitable but I'm not sure, yet, what powers would be useful for a ghost who was previously a diplomat/spy
So far, I have the following ideas
Masking (make the magus invisible as per Veil of Invisibility)
Phase Shift (make the magus incorporeal)
Prying Eyes (as per spell of the same name)
Preternatural Hearing (as per Whispering Winds Spell)
Kinesis (as per the example on pg 192)
I think he really should only have one power of cost 5 that can be used 2 -3 times a day. Do you think those powers would fit well with the concept?
All the powers except Phase Shift, IMO.
Making a body incorporeal is a base level 30 MuCo(Au) effect and as such might be a bit powerful to ascribe to might 10-15 spirit Warder.
Another possibility might be a power akin to Frosty Breath of the Spoken Lie which is Mentem based (InMe) and only level 20. This or the others would be very useful for discovering information (as per his spy abilities in life) or uncovering falsehood in others.
oooh. Now THAT'S a nice idea. A ghost who can reveal whether someone is lying...hehe
Hehe, just be careful House Guernicus doesnt find out about it or they'll try to recruit him away from you! 