League-of-legends – How big does Cho’gath get with Lulu’s ultimate

Any effect that scales off of maximum health takes the current health maximum of the champion. This counts for on-hit procs (bloodrazor) as well as targeted abilities (Fizz's trident) and AoE abilities (Xin Zhao's ult, assuming it's levied against someone who is currently at their max health).

Those abilities take into account items granting permanent health, items granting temporary health (red elixir, for instance), and, yes, Lulu's ultimate.

While it should go without saying, anything that adds a shield, but not health doesn't increase the calculation for maximum HP. This design choice allows the developers to change the power level of abilities and items in a more granular manner.

Short answer: Yes/Yorick will die, revive and Cho'gath will gain one stack.

Feast (Active): Devours a target nearby enemy unit dealing true damage. Feast always deals 1000 (+0.7 per ability power) true damage to minions and monsters. If the target is killed, Cho'Gath grows and gains extra health. This effect stacks up to 6 times and Cho'Gath loses half his stacks rounded up upon death. Cost: 100 mana Cooldown: 60 seconds Range: 100

Source

• The meaning of "the targer is killed" count as Clones, minions, neutral monsters and others.

When you feast a target affected by Yorick's ultimate, and KILL him, you will get stacks. He WILL revive after that for a short time. But you haved killed him.

• The only momment Cho'gath does not gain stacks are when he DOES NOT kill the target. Like when someone is with Zilean's ultimate or Guardian Angel.