When using the editor: For some strange reason, I am unable to resize fluids with [Left mouse button] + [Alt]. Funnily, I am able to change the height with [Right mouse button] + [Alt]. I tried it in both possible ways: Pressing the mouse button at first or pressing [Alt] at first.
Additionally, I also noticed I can’t use anything which uses a semicolon or an equals sign as a key binding.
I use a German QWERTZ keyboard. For a semicolon, you have to use [Shift]. For an equals sign, you have to press [Shift], too. (Maybe therein lies the problem.)