The same size as regular people.
When you think about all the formidable and terrifying implements of destruction an advanced-technology society might have at its disposal - nukes, bioweapons, lightning-fast tanks, rocket artillery, pocket-sized exploding spy drones, autonomous jet fighters, cyborgs with gatling gun arms - spending the extravagant amount it would take to genetically engineer supersoldiers just to get a result that's still fundamentally human is a bit underwhelming. Unless you take that liability and turn it into an asset.
There are two big advantages you can derive from the fact that your supersoldiers are human. One is that they're compatible with all of the infrastructure and technology you already made for regular soldiers. Need a super-tanker, super-pilot, or super-artillerist? You can, and without having to redesign your (fantastically expensive) materiel. Need to ship a unit of super troops to the front? Don't bother with the hassle and expense of setting up any specialist infrastructure (like you'd need with armored vehicles or aircraft), just drop them into the basic infantry barracks you already set up. Or do what regular troops do and "requisition" some handy civilian building.
When it comes to special operations, the advantages are even more profound. Partisans throughout history have found that one of their greatest advantages is to be able to take off their weapons and uniforms and blend with the civilian population. Unlike virtually any of your other advanced weapons, your supersoldiers could hide unnoticed in your enemy's population - or your own, if you found the need.
But, these advantages only apply to supersoldiers who are basically the same size as other soldiers. There's always a temptation to make your supersoldiers ten feet tall with refrigerators for torsos and giant ape arms, but when they can't drive a tank, hop a ride on an air cav helicopter, or even enter an apartment building without banging their head every five minutes, you'll see the disadvantages of that approach. Normal size is better.