YES, it is too big in my opinion. I'm 6'1" with proportionate hands. I've owned huge phones like the Lumia 1500 and currently own a Note8. Just because you can do most tasks with one hand on a big phone doesn't mean it's the most ideal or comfortable experience.
Keep in mind: It's not just about the width or height of the phone, how thick the device is factors-in as well. You should be looking at the overall volume of the phone to decide if it's the right size for you, not just individual height and width measurements.
The S8+ is not the most accurate measure of how the S9+ will feel. The S9+ will feel larger in the hand than the S8+, and is about as much smaller than the Note8 as it is larger than the S8+.
The increase in thickness to 8.5mm puts it just about on par with the Note8.
Likewise, the S9 will feel appreciably bigger in the hand than the S8, commensurate with an increase in both width and thickness.
My Note8 had freezing problems, so for the past several months I've been using a Nokia 8. It made me realize how much I missed phones that thin and generally comfortable to hold and use with one hand.
I found the S8 to feel a bit small and the Nokia 8 just about perfect. The S9 will be closer in volume to the Nokia 8 than to the S8.
For these reasons, I'm going with the S9. I took the liberty of calculating the overall volume of all the phones mentioned in this post to give you an idea of how large they'll feel with one-handed use.
Here's how they rank (in cubic millimeters):
1. Note8 - 104533
2. S9 Plus - 99176.13
3. S8 Plus - 94829.13
4. Nokia 8 - 88207.845
5. S9 - 86249.415
6. S8 - 81120.72
7. iPhone 6 - 63843.63
I, personally, would find the S8 Plus's volume to be on the max end of acceptable for one-handed use. If the S9 Plus was the exact same size as the S8 Plus, I might have gone with that. But my experience with the Note8 tells me the S9's volume is much closer to that ideal in-hand look and feel.
Whatever you decide, hopefully the measurements in this post help.
*EDIT: Added the iPhone 6's volume for the OP. One more thing to note, You're already getting a 5.8-inch display with the regular S9... we used to need huge phones for that display size. That's no longer the case with bezel-less displays. You're already getting a huge increase in display size over the iPhone 6 with the regular S9.