This question is oft-asked, and there's no really good answer.
1. Some listings on B&N give a page count. Some don't. That page count is not the e-book page count, but a page count of a paper version. "Pages" on e-books are a rather nebulous concept and don't have a particular size, but usually are larger than printed pages so the page count of an e-book is usually lower.
2. All listings on B&N give a file size. For EPUBs (which almost all NOOKbooks are, especially for NOOKcolor), it's pretty much guaranteed that the number of "pages" will be less than the file size in KB. So a 230 KB file won't have more than 230 "pages".
3. If there's a paper version, you can check the number of pages in the paper version yourself, but see item #1.
4. If the e-book is from Smashwords, you can go to Smashwords and check the word count.
5. Otherwise, check the reviews. Usually if an e-book is small, someone will remark that it's a short-story or a novella or just an excerpt or whatever the situation is.
I went back and checked the file size on the book I ordered and it was 1978KB, Since the number was high I just expected it to be a good book with lots of mystery. Boy was I wrong!! There were no customer reviews, maybe that was a sign...don't read.
In your case, the file size was probably an indication of graphics within the text. Sometimes the publisher will use a small graphic at breaks in the text or the beginning of chapters. Sometimes they'll even use a graphic for the chapter titles so they can make it use a special font. Graphics in the text or even a high resolution cover images will drive the file size up a lot. Another thing you can do is to check the reviews at Amazon as well. Their customers will complain just as loudly as B&N's about buying a novella at full book price.
There were no customer reviews
Maybe you should add one!
You can always look on Goodreads but a lot of that information is incorrect also.