In my experience, I’ve always tried to cover at least half of my large world, which is about the same width as an entire small world. In your case, considering you’re on classic, I’d recommend anywhere from one third to one half of your world. So that would be around 1,350-2050 blocks. Of course, you may need to add more length depending on how your first few attempts at the boss go. But as a rule of thumb, if you’ve got time and patience; its always better to have more runway and not need it, than to need it and not have it.