Open your blueprint in a text editor (it's a simple text file) then search for the engines you want to scale up. It will look something like that:
In the highlighted section, change x to stretch your engine horizontally, and y to stretch it vertically. For example, using a value of 2 will double its size in the concerned dimension.
Also, those values can be negative, it's the case of my x. It simply means that the part is mirrored horizontally or vertically.
If you have to modify many engines, I recommend doing it with a single engine, save your file, open it under SFS, and duplicate your engine as many times as you want.
By the way, you can also customize the size of your fuel tanks. It can help to make a few giant fuel tanks, that will look nicer and that will be easier for your game to handle. That's what I did here for example: