I think what I do may help you.
First pick a real-world item that you can use as a relative scale. Since the games I like to play with usually have human characters, a 6ft/72inch human is a pretty good point of reference.
Based on that human, other measurements are fairly easy to guess. Import your reference image (the human) into Construct 2(or any other game engine) and see how it stands up. Is it too big or too small for the screen or viewport that you want? If so, adjust that model until it's what you want.
From that, the other objects you make that are a relative size of your human will be spot on (or very close!) to what you want them to be when you import them.
For software and vectors I just use Inkscape. It's free if you want to try it.
I hope this helps.