It's hard to describe because there are so many variables, it's different every time.
Size, speed, force, angle of penetration, amount of lube, and most of all, chemistry between partners all play a part in the experience.
I've had monster cocks that just slid right in with ease while some smaller one's felt like they were tearing me apart.
For me, there are two types of fucks, good ones and bad ones...
The bad ones...
It hurts so bad you just want it over. It doesn't matter if it's a huge cock pounding you into mush, or a smaller cock going slow and easy. Sometimes it just doesn't feel right. Pure agony...
The other bad fuck...
You don't feel anything but uncomfortable, it doesn't hurt at all, but there's no pleasure either. And your thinking to yourself, Are you done yet?
Then there are the good fucks...
Good fuck 1...
It feels like a sensuous massage from the inside out. You almost want to go to sleep, not because it's boring, but because it feels so relaxing and comforting. And it doesn't matter if it's a huge cock pounding away at light speed or a smaller one going nice and easy, it just feels so right...
Good fuck number 2, the best fuck...
Every nerve in you body is tingling, all you want is more. Harder, faster, deeper. Your clawing at the sheets or his back, you legs are wrapped around his waist, trying to pull him in deeper. You can't see or hear anything besides the two (or three) of you, your in sensory overload and you never want it to end. Your literally being fucked senseless, and this is as "right" as it's ever going to get...
But again, there are the variables. I've experienced all the above with the same partner at one time or the other. We weren't lovers, just fuck buddies and he had a nice, thick 8 inches. Most of the time the sex was the mind blowing, sensory overload, fucked senseless or sweet relaxing screw. But there were a few time when for some reason or another, it wasn't right. It all just depends...