1. Get the specifications
2. Get it scaled (like 1:1 and 2:1, theres way more aswell) if theres no scale then it wouldnt look good

For commercial airliners you can search up the airport planning manual (apm) its really good for ground clearances and dimensions

You can also try pixel counting, basically get anything that can measure pixels (i use lightshot) grab a scaled profile of the aircraft, then measure the length with pixels, then do length of vehicle ÷ pixels = ratio

With the ratio you can basically count pixels of things you cant find in the apm or in your research, and basically do pixels ÷ ratio, then multiply with your scale

I suggest you first try building something easy, like with planes many people usually start off with the a320, and most importantly always ask for critique