One of the main ways to make your build as realistic as possible is to scale it. This basically takes the real-life size of the thing you want to build and convert it into Minecraft dimensions. This is a tutorial on how to scale YOUR builds!
You can also use this to make buildings and other structures.

When scaling in Minecraft, there are three things you need to do:

First: Find your dimensions. Whenever you find dimensions, make sure the measurements are in meters, not feet. One Minecraft block is supposed to be 1 meter in real life. You will need the Length, Width and height of the vehicle. The best place to find all three dimensions in on Wikipedia. Search your vehicle on Wikipedia and scroll to the bottom. Around there are all three dimensions. For aircraft, the Width will be labeled as "Wingspan". This is just another word for Width. Wikipedia also normally gives the dimension in both Feet and Meters (m/ft). If the measurements are not in meters, simply use a converter to find how many meters it is. As an example, we will do the F-15 Eagle. Make sure you get the measurements to the nearest hundredth or tenth, if possible.

F-15 Eagle dimensions in Meters:

Length: 19.43 meters
Wingspan/Width: 13.06 meters
height: 5.64 meters.

Second: Now that you have your dimensions in Meters, you want to find out what scale you want to do. The three main scales that most builders use are 1:1, 1.5:1 and 2:1. 1:1 is the easiest to do for finding the measurements-. You will simply take the Measurements you already have, and boom! Those are the dimensions of the Aircraft in-game. For 1.5:1 and 2:1, you will need a calculator more than likely. 1.5:1 is 1.5 time larger in Minecraft than it is in real life. This give you a decent amount of room for details, but it also requires more work on shaping. For 2:1, it is twice as big as it is in real life. This gives lots of room for details, but a lot of attention is required to make them. To find these scales, simply multiply your dimensions by the first number of the scale. (2:1)

F-15 in scales:

1:1
L: 19.43
W: 13.06
H: 5.64

1.5:1
L: 29.145
W: 19.59
H: 8.46

2:1
L: 38.86
W: 26.12
H: 11.28

Third: Simplify! All you have to do now is simplify your answers. For length and width, round to the nearest block. If your answer has a .5 or above on it, round up to the next block. If it has a .49 or below, round down. Your Width should preferably also be an odd number- if it isn't, round to the nearest odd number. for your height, you can be more flexible. I would round to the nearest 1/2 a block. If your number has a decimal anywhere from .3 to .75 on it, I would round it to .5 and make it a slab on the top. If it is from .2 to .3, you should be able to use a trapdoor. Anything below .2, I would round to the nearest block. I would do likewise with anything above .75. These new numbers will be your dimensions in blocks!

F-15 in scales, rounded, in blocks:

1:1
L: 19
W: 13
H: 5.5 (5 blocks and a slab)

1.5:1
L: 29
W: 19 (The Nearest odd number is 19 in this case)
H: 8.5 (8 blocks and a slab)

2:1
L: 39
W: 27 (Nearest odd number is 27)
H: 11.2 (11 blocks and a trapdoor)

Now, simply put these dimensions into your Minecraft world, and those are the dimension of your vehicle! Height will be vertical, and Length/width will be on the ground.

The Image attached Show you how to put your dimensions into Minecraft.
The iron blocks show the Height of the vehicle
The gold blocks show the Wingspan/Width of the vehicle
The diamond blocks show the length of the vehicle.

I hope this was helpful for you, aspiring builder. Thank you for taking some time out of your day to read this :)