The general rule of thumb is not to write or rewrite your novel to fit any one given medium (this doesn't apply to all forms of written works, but we're talking about novels.) Instead, edit it to fit, which is what would happen if Tapas commissioned you to write for them and you gave them a work that they liked but didn't fit their delivery system.
That said, pacing is important for any novel and I'll give my advice on your chapter word count. There's no hard rules for word counts, but too long of chapters dissuade readers in today's age because we often don't have a dramatic amount of time to read. The average young adult reads in one or two chapter bursts throughout the day, and having 2,500 word chapters (which is roughly what 15,000 characters correlates to) is pretty large, that's about 5-6 pages. The average novel works with 1,500 words, or 3-3.5 pages. Some get away with more, some get away with less. Personally, my chapters are usually smaller in the first draft, hanging around 800-1,200 words, and only the larger ones end up around 1,500 words in final drafts. But that's just my personal writing style for novels, and my short stories tend to be exceptionally long because I employ a different style for writing them than my novels, so you can see how subjective it is.
But, it's entirely possible that your novel works better with large chapters - or perhaps the first chapter by itself is just longer than average, which happens and isn't a problem. If that's the case, edit it to fit the Tapas character limit, perhaps by marketing two separate posts as "Chapter 1-1" and "Chapter 1-2", followed by chapters 3, 4, etc.
