In the help ist says:

Resize the tank

1.

In the network editor, go up to the Scene level, then double-click the fluidtank_initial object to go inside.
2.

Click the fluidbounds node.
3.

Use the handles in the viewport or the parameters in the parameter editor to change the size of the box.

Num 1; Wrong. Inside the fluidtank_initial, there is a fluidtank node that holds the fluidbounds node.

Num 3: The parameters in the fluidbounds node are locked, one can only use the viewport handles.

Why are those parameters locked in the first place? I

whenever parameters are locked, you can right click on the parent node and click ‘allow editing of contents’ to unlock them.

sometimes if you are a few nodes deep, the locked node may be the first one, so if the option is not immediately available, go up another level