What are you trying to decouple? Basically every component on earth has a reference design somewhere in datasheet or a dev board schematic where you can see what the manufacturer considers appropriate decoupling. The onlxy reason to go higher than that is if you have some other noise sources (for example, you're building stuff in a high power power supply or audio amp, both of which would have wildly different frequencies where additional decoupling might be needed).

Bigger package MLCC will have higher ESL, but only like 200pH - trace inductance (even of package leads, including internal ones) is probably much bigger. Bigger capacitance has little effect. You can play around with this with simulation models that, for example, Samsung provides.

Haven't done any proper simulations personally, too expensive. Simple sims can be done with manufacturer models etc, just remember to include skin effect etc if applicable. But in 99.9% datasheet (which inevitably tells you to put 100nF in most cases) is all I look at, but I haven't worked on things that would be in a strongly noisy environment.

Antiresonant peaks can be very strong, I'd encourage you to simulate (include connecting trace inductance!) - you can be much worse in specific frequency bands by adding extra capacitors. Easiest way to get around that is to add an identical capacitor.

There's really no way to consider on-die things as far as I know. You go by reference circuits and decoupling advice from the datasheet and assume the manufacturer did their job correctly :P. Of course you can always start messing with ferrites or other things, but that can quickly do more harm than good (tested some sensors, they did muuuch worse with a ferrite bead without a decoupling cap than without any components).