I will give you a impression from the buiding of my Volvo EC700B Excavator.
It take's me more than 3 years to built.
Scale 1:16, weigth 22 kg, length with boom more than 110cm.
The first thing I do is buy a decast model 1:50, find drawings, pictures and details of the real one.
I scale up the PDF file from the Volvo site and use this to get the dimensions then I made construction drawings in Auto Cad.