Length Contraction: Why Must dx be Measured in the Prime Frame?

[Pezz]

The Lorentz transformation for motion in the same direction along the x-axis of two frames can be used to derive the length contraction formula. Say we are converting from dx to dx'. The t would obviously have to be 0. That would leave us with dx'=gamma(dx). My question is why dx here has to be the length measured in the prime frame? If we do pick an object in the prime frame we get a longer length that the prime observer measures. However what happens if we pick dx to be an object length in the unprimed frame? Then again it turns out that the primed observer would get a longer measurement but I know this isn't correct. I'm just trying to find the reason as to why dx has to be the length of an object measured in the PRIME frame with a relative velocity and why it cannot be a stationary object.

 

[Pezz]

I mean why can I not measure the object instantaneously in my own frame and enter it as dx.