I'm trying to decide whether to choose a trim size of 5.25 x 8" or 6 x 9" for my 100,000 word fantasy novel. I was leaning towards 5.25 x 8. My novel is a Kindle e-book and I'm looking at converting it to paperback with Createspace. Presumably a larger trim means less pages which may mean a slightly cheaper printing cost? Many thanks in advance for your thoughts. :)
There are a few things to take into consideration for trim size.
I have had experience using CreateSpace as well. Published a book with 90,000 words, and that was in a 6 x 9 trim size. They recommend this size as the standard for market. My 90,000 word novel cost about $4.50-$5.00 USD (with color cover) to print. My next novel is going to be published with CreateSpace as well, and it is 110,000 words. Seeing that you are right in between those numbers, I'd say 6 x 9 would be the best option for you.
If you're looking to see the price differences, you can find the CreateSpace print cost calculator here: Printing Costs (Then click on 'Buying Copies' tab )
For a 5.25 x 8 Trim Size (300 Page Book Estimated size based on my last count and your # of words) would be about $4.45; and $4.09 for a 6 x 9 trim size. The cost of printing would only vary about .40 cents per book. Usually the cost goes down with the higher quantity you by (but this is due to shipping costs).
So to answer your question, your cost of printing would go down around fifty cents if you chose the larger 6 x 9 trim. CreateSpace recommends this as mentioned above: that is the industry standard for books this size, as it gives you more options for distribution. As for kindle, they can format it well in any size. The main concern you should have for your trim size is the width of your book's spine. Going with the smaller trim for a 100,000 word novel would lean toward the thicker side. CreateSpace also has a formula to calculate this. Spine Width
Overall, make sure to really research all your questions about CreateSpace before you move forward on publishing. You may want to see their community forum for more info. Congrats on your accomplishment, good luck!