It seems to me that if you can't use Human Guise on a Power while using that power, then you can't ever use it on a size power. When are you not using Diminutive Size?
Technically that is correct - the powers can not be used while the Human Guise is "active". It is rather simple when it relates to "positive" Powers, but in this case it becomes problematic as it is a Power that in many ways is negative.
In OW, the ones with Human Guise are the WCV (easy), Hecatian Hags (also easy, man sized), Ghouls (same here) and most of the Denarians. In the case of some of Denarians, these seem to be the only ones that have Hulking Size in combination with Human Guise (no one in OW have Diminutive Size and Human Guise, perhaps that is a clue to the fact that they are not supposed to work together?). This would technically be the same or at least similar to Diminutive Size, in that Hulking Size also have a number of "negative" sides - these negative sides should not be affecting the character when in Human Guise (he is not hard to hit, Stealth is not considered Mediocre etc).
If Human Guise would only work on persons of a size relatively close to a standard human being, it would be fine (but the Denarians seems to be an exception to this).
Glamours or Human Form might be better here.
I see in OW that Ogres (Hulking Size) and Pixies (Diminutive Size) do have Glamours, most likely used to hide their true form. This is probably the best way to do it.
EDIT: I do not want the player to use Human Form, as the Fey are not really shapeshifters.