Johnny-jay is right that one is much more likely to hear the phrase "that's a big maybe" and much less likely to hear of a "small" or "medium" maybe. While it's true that a "bigness" of a "big maybe" could be referring to the likelihood of the incorrect spell working. (in this interpretation, the "bigger" the maybe, the lower the chance the spell will work. However, it's important to read and understand the issue being disputed between Holly and Tara. Holly thinks that what is important is to have the right "energy and intention" and to hope that the goddess will know what they mean rather than worrying specifically as to the words that are said. Tara is upset because she is worried that the wrong words will result in the wrong spell with possibly disastrous results (e.g. a plague of locusts). So the "bigness" of the maybe is not just about the likelihood that the spell won't yield the intended result (i.e. breaking another spell) but also about the risk that it will yield a totally unintended and disastrous result (e.g. a plague of locusts). Imagine you are standing on a dock as a boat is leaving and you say to your friend "maybe we can jump and land on the dock". Your friend looks at the distance between the dock and the boat and then at the water and says, "Yeah... that's a BIG maybe." That is... he thinks there is a small certainty of getting the desired result (i.e. being on the boat) and a much bigger likelihood of getting an undesired result (i.e. being in the water). Hope this helps.

"That's a big maybe" is an expression. In this example, Tara is saying it is a "big maybe" because she doubts that the goddess will know what they mean. Personally I don't use this expression nor do I hear it a lot. You would use this expression as a response to someone saying maybe something will happen, when you believe it actually has a small chance of occurring. I have never heard of a small or medium maybe. I hope this helps :)

Hi Ai, what she means by saying "That's a big maybe!", is that it will not happen. Or, it is unlikely to happen. If she says the spell wrong, it is going to be wrong. For the spell to work - it must - be said correctly.