How do I define the fitness of a fungus?

The size of it's underground part?

"Fitness" for what? They are millions of years old, I would say that that is adequate evidence of fitness. But perhaps you mean something else by fitness. Be a little more specific.

You seem to be describing the resistance to fungicide. There isn't a scale on which you can measure resistance. It depends on how good the fungicide is. Some fungi will survive, they are 100% resistant; the non-resistant ones will die. There is no such thing as 50% resistant.

How would you measure the "resistance" of humans to cancer ? Some people might survive with it for a period of time, but in the end all will die. Once you've got it, you are stuck with it. "Resistance" would mean that you don't catch it in the first place ! There is no way of measuring "resistance". Nor can you measure it for plants against weedkiller. Or for fungi.

I'm imagining you're wanting to write about how fungi have adapted to survive differently from other organisms.... can thrive without light, likes moist/damp surroundings, and so on. See "Source", below.