Suppose you are told that the linear size of everything in the universe has been doubled over night . Can you test it by using the fact that the speed of light is a universal constant and has not changed ? What will happen if all the clocks in the universe also start running in half speed?

Hi,

Yeah it can be tested remembring that the speed of light is constant.
Suppose you measured the time taken by speed of light from one place to another in normal world, in the altered world although the length will be doubled but the time is still running at the same pace, so light will take more time to travel the same distance in this way we can observe that the length has been altered.

Now if the clock also starts running at half speed then there will be no time difference in both the cases and hence we wont be able to observe the change.