Here is a quick tutorial on how to get the shape right and the size
Let's just say you want to build a replica plane

First step: start a new project just a cockpit and the blocks beneath it
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Second step: then put a Fuselage under the starter cockpit then stretch it to the longest length possible (10) and add more Fuselages to it until its the same length of the real aircraft you want to make(you can know by clicking the 3 stripes then the info button then check the length, adjust it by ft or meter as you like it)
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Thrith step: then put a Blueprint Then put at the side view and resize it till its the same length as the line we made earlier
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Forth step: then put an others Fuselage at the blueprint we just installed then continue stretching it and keep adding short sections to it until you have the shape of the plane you want (^^)
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(It can also work with a blueprint you make or anything simple sketch of a plane)
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I hope that helps (^^) if you need any clarification or my words were a bit complicated. Please let me know,

Blueprint always be your best saviour