are you using the quick reply box?
It will add lines as you go too but maybe it has a limit.

or the normal reply to thread button?
If your post is too large it will tell you when you submit the reply, and tell you how many characters?? you have.

I hit & held down the enter key in this post to see how far it will go and it just kept going- you might try an experiment like that to see what happens on your computer.

Of course you'll have to "delete" all those invisible "lines" before you submit the post.

you shouldn't be having any new pages ?/ when replying to a post - it's all in one page/box.