About 98 feet, give or take
Obviously, there would need to be a few changes to survive in the water, though I will keep to your requirement of 'partially human in shape'. Completely human is fully impractical, and I will stick to the requirement of 'realistically evolve on Earth'. Alright, let's start.
The first thing we do is make a few adjustments to the human. Independent legs are tossed aside right away - we're combining them to turn them into a single tail, and as long as we're doing that, we're going to adjust the feet so we turn this into a proper fluke tail. Might as well. The next thing we do is subtly adjust the method of getting oxygen to the lungs - we moved the nose to the creature's back and play around with it so that it becomes more of a 'blowhole'. Then we make adjustments to the creature's arms and make them as finlike as we possibly can. Obviously, since we're going for a realistic evolution, we're then going to want to streamline the whole creature to make sure it can cut through water as much as possible. Lastly, when enlarging it, we can't do it uniformly, thanks to the square cube law - the legs and arms will have to appear small to the enlarged torso, but if you want realistic, it's the path you're going to have to take.
And what you're left with looks remarkably like a Balaenoptera Musculus, except slightly more humanoid. I mean, it's not that humanoid at this point, mostly thanks to the streamlining we did in order to make it realistic in the water, but the skeletal structure closely resembles that of a human, and that's what counts.