The basic consensus on what the GM should allow in DW seems to be anything, as long as it fits with the fiction.
This results from the GMs principles and agenda, especially be a fan of the characters and play to find out what happens.
So the first question the GM should ask oneself is: Does this make sense in the fiction?
Taking your example of the Black Blood Death Disease (BBDD) I would ask myself (and the Cleric as well!) if the Cleric's diety would approve of the Cleric using their magic in this way. If it is the general consensus that the diety would not disagree with the use of the spell, there's probably no other fictional hindrance preventing the particular disease.
Let me now address a few of your specific questions.
Does this spell basically mean that, if no diseases have been established, the cleric gets to make up a disease on the spot? - Yes, especially since DW encourages players to participate in would building.
Does the cleric get to determine what effects that disease has, or do diseases roughly translate to debilities? Might a disease cause several debilities? (Probably). - The mechanical consequences result from the fictional description of the effects of the disease.
And finally, making an assumption here, how can the GM rein in the "created" disease to ensure that it isn't an "instant win" spell for the cleric? - By filling the character's lives with adventure! "Well Cleric, I guess a horrible disease like the BBDD would surely be dreaded. What precautions are needed not to get infected yourself? Is there any treatment in case an innocent soul got infected?" Maybe at some point later you get the opportunity to reveal an unwelcome truth/turn their move back on them: "As the ranger misses his shot and the arrow buries in the belly of the Goblin killed by the BBDD, the body is dismembered in a violent eruption and a cloud of dark black gas is radiating outwards toward the Fighter, who is locked in melee with the Goblin King. Cleric, what do you do?"
And a few weeks after, word may have spread and people in the city may be whispering when the Cleric passes by, sounding murmurs about the horrible disease that was spread in the name of their diety.
Summarising, I suggest a permissive handling of player input with thinking dangerously about all sorts of possible side effects the Cleric may or may not have reckoned with.
Best Answer
Each spell tells you what needs to be rolled, in its description.
The size of the shield is not specified (and doesn't really matter), only its effect: +5 to AC and immunity to Magic Missile, until the start of the caster's next turn.