RosieCheeks:
My 'two pennyworth' on the matter is personally for me as a reader a level of realism is essential, as a good story draws me into it and yes i like to feel almost part of the story, to feel the emotion etc, so a statement of can't sit down after a moderate hand spank does put me off reading it.
Context absolutely matters and in what you describe here, I would tend to agree with you. "Can't sit" is not a credible statement after any hand spanking I've experienced or could imagine. There probably are some people who have made that sort of statement after such a spanking, but unless the story is focused around a character who is humorously overstating everything that happens, I can't imagine it to be much of a good read. I do think there is a middle ground between this and a devastating beating where the "can't sit" statement has an acceptable place in a realistic story.
There is a wide range in the potential severity of a spanking involving an implement such as a paddle, strap or cane. When my sisters and I were teens our dad would on occasion use an oak paddle to discipline us. He typically gave us twice our age in swats which meant somewhere between 26 and 36 swats. Those spankings were some of the worst we got at home. For me the discomfort was completely gone within a couple of hours. My sisters often complained their bottoms felt bruised for as much as a couple of days after. None of us had any visible marks beyond a few hours after the paddling occurred.
We were also subjected to paddling at school with a nearly identical paddle. Dad bought his from the high school wood shop when my oldest sister started high school. The spankings we got at school were less swats than we would get from our dad, but the results were significantly more severe. The swat count was usually between 10 and 15 depending on the offense and our disciplinary record at the time. I think I got the worst of the three of us when I was paddled with 25 swats for fighting.
For me, the discomfort lasted several hours after the spanking while my sisters complained for several days. We had visible marks and bruises that remained for anywhere between a few days and a couple of weeks, depending on the severity. It took around 10 days for my bruises to disappear from that 25 swat paddling.
In this way it's clear to me that there can be a wide range of interpretation for the severity and results of some spankings. I've read stories where 50 swats with a paddle are given. In comments about those stories I've noticed some think the number of swats and consequential severity is beyond anything reasonable or realistic.
In the context of a spanking from my high school principal, I would agree the severity of that spanking would be beyond reasonable and, while still technically possible, it would fall outside the realm of anything I want to read about. On the other hand, in the context of the spankings my dad would deliver, it wouldn't cross over into that abusive amount of severity. He simply didn't wield the paddle in such a way that it was going to leave any long lasting bruises or marks. I suspect he could have delivered 100 swats and still not matched the severity of 10 from our high school principal.
Every story I've read has some room for subjective interpretation. Our life experiences often have a significant influence on what we consider realistic versus fantastical. When it comes to exaggerations and non-literal statements like "can't sit" I find the context is all important. In some stories it feels natural and works and in others it sounds out of place and destroys any sense of realism. It's all in eye of the beholder and no one's opinion on the subject is wrong.