Adult fiction has a safe zone of 70,000–100,000 words, whereas adult nonfiction has a safe zone of 60,000–90,000 words. Less than that, and your book will most likely be too short. More than that, and your book will most likely be too lengthy.
So you're lucky to have about 100,000 words to tell your story. That's a third of a million words. It's not even half the length of a typical novel. But it's enough to feel comfortable about whether or not you've told your story.
If you want to write a full-length novel, try writing one first. See if you can get it done within the word limit you've been given. If you can't, go back and cut some things out or change some other parts of your story.
The number of words you use isn't very important as long as you use them all up. You could write 10 books with only 20,000 words in each and they all count equally as novels. As long as you cover all aspects of your story and leave room for readers to imagine what might have happened after you finished typing, you're good to go.
However, word count recommendations fluctuate according on genre and age group. As previously said, general fiction for adults should be around 100,000 words. However, genre works (fantasy, science fiction, or historical fiction) are frequently lengthier, clocking in at roughly 125,000 words. Young adult novels fall in the middle at about 112,500 words.
In terms of history, it would depend on how in-depth you want to get with each period. If you were writing about World War II, it would take longer than if you were writing about the French Revolution. The more detailed your description, the more words you'll need.
As for time periods, it really depends on your audience and what they're interested in reading about. If you write about an ancient Roman then you'll need more words than if you wrote about modern-day America. But if you write well then your audience will want to know more and more about the characters and the world around them as you go along.
Length is just a number and not every book has to be long. In fact, some books are better off being short because there's no way to cover enough ground in a short space of time!
A book that is published in paperback format and has a length of 300–400 pages would typically have between 80,000 and 100,000 words. 100,000 words is a decent goal for adult fiction (particularly crime, thrillers, women's literature, sagas, and romance). Anything longer or shorter than that is unusual for a first book.
The number of pages a novel contains depends on several factors such as the size of the typeface, the quality of the paper used to print them, and the level of detail included in the artwork. A page usually consists of one side of a sheet of paper, which at present time is usually 8 1/2 by 11 inches (or 215 by 279 mm). However, pages can be any size as long as they are equally divided into text and illustration areas. For example, a page could be half as large again as usual, or even smaller if both illustrations and text fit on it.
Pages should be numbered so that you can refer back to them easily while writing. Some people write out page numbers in the margins of their books, but this is not necessary. Just use numerals or letters if that helps you keep track of where you are in the story.
It is best if you can get feedback on your manuscript from someone who has already written a book of some kind.
If you're creating a young adult fantasy novel, limit the word count around 90,000 words or 300 book pages. This is because the audience you're aiming for prefers that length. If you're writing an adult fantasy novel or an epic fantasy novel (like Game of Thrones), your word count should be in the 90,000-200,000 range. For science fiction or horror, go as long as possible.
Your story shouldn't be too short or too long. Aim to write around 20,000 words if you can first draft it in a month. A longer novel will take two months to complete or more. You need to allow yourself enough time to write a good story!
There are lots of factors to consider when putting a number on your novel. The most important thing is that you have fun with it and don't worry about what other people think. Write what you want to read and keep it within these limits - otherwise you risk turning off your audience.