Ouch. Too many different perspectives. i agree with PypeBros, figure out the dimensions of a simple cube, and then make the rest of your elements match the ratios of height to width to depth. I don't think your tank even matches itself (wheels vs hatches on the back). Time to break out the math!
I think this is it. For a cube of side X, the height squared + the depth squared will equal the width squared. Note: this only works for cubes that face the screen. For an angled cube like a typical iso view, the width and depth are diagonals across the top face of the cube so the math needs to account for that.
The good news is that the numbers don't have to be perfect. Looking at a sample spaghetti crate (or is that pizza?), I see dimensions h = 7, d = 14, and w = 16. 7^2 + 14^2 = 245, which is close enough to 16^2 =256 that it works ok. But keeping a ratio of 7: 14: 16 across detailed objects like your tank is difficult.
Maybe use a different ratio of numbers that you like better. Anything with less than about a half pixel error should be ok, I think. Just keep it consistent across the image.
Hope this helps,
Tourist