I have sent iMessages with thousands of words at once without issue.
It seems to me that the sender is using regular SMS instead, which has its own limits.
Make sure the sender and you both see blue bubbles instead of green ones when texting each other.
Assuming the person(s) sending you messages are also on iOS, there shouldn't be any issue sending lengthy iMessages. My wife, son and I iMessage all the time and sometimes I type out novels. They've always received all the message.
But as @Tartarus and @bamf-hacker said, have them check their settings. If they are sending via SMS then there's nothing that can be done as that's a restriction with SMS.