There are ones with smaller sizes, but the reason they're huge is so they don't fall off easily. That's why they're called plugs.
Don't immediately get one and just expect to shove it up willy-nilly, you need to prepare yourself and experiment. Learn how to loosen yourself first before you decide to buy that. Be sure to get lube when you order it.
Also... Learn how to douche if you don't want a literal shit show the moment you pull it out.

Tbh the most unrealistic part of the manwha is how jiwooks anal fissure healed. I just got done shoving a suppository up my ass because I thought a rectal cream was enough…but NO! Apparently if you don’t get tiny buttplug pills as first choice of treatment u have to get ur asshole examined by a middle aged man (I usually get taken out for dinner first) because it didn’t WORK