Open creature_standard.txt in your dwarf fortress\raw\objects directory.
Edit the body sizes of goblins, elves and humans so they're 2 (5? 10? 50?) times their normal size, by messing with the numbers in the [BODY_SIZE] token.
Note that there are 3 bodysize tokens, you really only need to change the bottom one, which is for their maximum size. The others are their average size at birth, and their average size at 1 year and 68 days. It can't hurt to change those too, but I don't think it's required.
Their equipment should upscale with them. Not sure if there's a limit to that though.
Fiddling with the various [PHYS_ATT_RANGE] and [MENT_ATT_RANGE] values can also help make them extra strong, agile, spatially aware etcetera.
I do think that if you set those too high that you will get completely unbeatable enemies though. Making them bigger makes them more dangerous but still killable, making them unbelievably agile and strong and inexhaustable makes them both dangerous and unkillable. Tweak with care. The multiple values used here represent a bell curve around a median. Looking at the values for the human race will give you an idea of what is considered average amongst races.
Give them [TRAPAVOID]**, [NO_STUN] and [NO_FEAR] as well
More experienced modders can probably tell you how to give their civs access to steel too.
Now go to war with elves and humans (and goblins if for some reason they're not sieging you yet), and have more fun.
I do think you will need a new world, at least to get enemies with the new body sizes. I do believe you can add the trpavoid, no stun and no fear tokens without genning a new world though. All new units entering your map should have the new tokens.
However, if you want to try that on an existing world, you need the creature_standard.txt in you DF\data\save\raw\objects folder instead of your DF\raw\objects folder.
Helpful wiki page for aspiring creature modders:
http://dwarffortresswiki.org/index.php/DF2014:Creature_token**) note that trapavoid still leaves them vulnerable to lever/pressure plate-operated spike traps, and webbed traps. Giving them [WEBIMMUNE] fixes the latter.
EDIT: There used to be this mod that had all kinds of more dangerous, larger goblins. I believe it was called Ender's mod.