The creepe myrtle should be transplanted now the plant is most likley going dormant & will take off next spring.
Once roots have formed the plant is able to take up water & nutrients & go on its own so length is not important... It should be noted that the larger the root mass is the more roots that will be damanaged in removing from rooting medium & transplanting.

"The creepe (sic) myrtle should be transplanted now the plant is most likley going dormant & will take off next spring."

I disagree. My crApe myrtles here in zone 6 aren't even close to going dormant. Wait at least a month for better results. In zone 7, you can safely transplant anytime the ground is not frozen between leaf drop and early spring.

"Once roots have formed the plant is able to take up water & nutrients & go on its own so length is not important..."

I disagree again. The cuttings should have sufficient root structure to supply enough water and nutrients to the cutting. If roots are just beginning to form, transplanting too soon can result in a dead plant due to transplant shock and the cutting's inability to get enough water and nutrients to overcome the transplant shock and grow.