Well I guess the simple explanation for the first questions is this: If the antenna is too short, it won't receive the entire wavelength. If it's too long, the radio waves that it receives will cancel eachother out.
The problem with unshielded twisted pair (and shielded also) is that the characteristic impedance is not uniform, so you can't get a proper termination. At low frequency it works OK but for high frequency, use coaxial cable.