Boy this is a tough one!
I intend to have a fullsize Boba Fett mannequin standing next to a full size Jango Fett mannequin.
For Boba Fett it was easy since Jerremy Bulloch was quite slim and a standard shop mannequin is about 6' tall, 38" chest, 32" waist.
I had intended to make my Boba and Jango the same size and use the same sized mannequins. Having them both side by side the same size would probably look best.
Its going to be near impossible to find a 5'7" mannequin and Jango will look like a dwarf next to his son!
Still not sure how to procede.