cojo965 said:
Q: What planetary conditions would be needed for actual kaiju?
That depends on the Kaiju, and what it can do do. If you mean Mothra? Magic. Godzilla minus the breath weapon? You'd need a LOT more free Oxygen in the atmosphere, aka Dinosaurs.
That's really the only practical limit, up to a point. You can get really VAST creatures, if the atmosphere is so rich in Oxygen that their circulatory system can keep up. You see this most rapidly reflected in the size of insects, but there is a point after which they can't survive.
Remember, that volume increases by ^3, but area increases by ^2. Nothing can get MUCH bigger than say... a very large dog without active respiration.
Once you have the oxygen, you need to have an environment that either provides enormous quantities of vegetable matter to graze, or plentiful prey. Fortunately the condition that would tend to support giant creatures would tend to support a huge supporting biosphere.