It's lovely to have an accurate expectation of the size of the book or product you're designing but designing on a PC can often be somewhat misleading when it comes to real-life size.
To understand an idea of the size of the product you're making, I recommend checking the measurements on the product description or the pricelist online.
You will also see the physical measurements of images on the page when designing in the RapidStudio software, which you can sketch down on paper with a ruler to have an idea of how big it will be on the printed page.
When zooming to 100% in RapidStudio, the product will be at it's "best fit" in the software, this is not an indication of it's physical size in any way.
Because your computer doesn't really know how big your screen physically is in real life, it can't portray things in real-life sizes on the screen.
It's recommended you have a ruler handy when designing just to look and check at how big things will be when printed.