To quote a Staff answer:
There's no explicit limit. It's not uncommon for people to publish posts that are many many times larger than that.
Depending on the style of your blog and content it might be sensible to break larger posts up using the --more-- or --nextpage-- tags, or splitting them up across several posts. But the reasons for doing that are aesthetic, not technical.
I would personally suggest you take a look at offline editing apps when writing yours:
Writing on browser can be fickle from time to time, and it's better to write elsewhere and then post it to WordPress once it's done.