Two popular POD providers, Lulu and Createspace, offer very different trim sizes. Lulu offers three categories: "Value", none of which are available if you want to offer your books through retail outlets; "Standard", which vary; and "Premium" which mostly aren't but do offer a variety of odd sizes as well as calendars and colour options for interiors. In the process of looking at this, I've just discovered that the options on offer depend on which store you've set as your default. In the UK store the standard category gets you A5, pocketbook, A4, and square. The US store, on the other hand, offers US trade paperback and US letter sizes and square. On the UK store the measurements are in centimetres and in the US store they're in inches, so it helps to know what size you want and how it translates across.
In addition, if you change store during the creation of your book or revision of your book you might get a size you weren't expecting. I formatted an A5 book, but somewhere in the process I switched to the US store to take advantage of the exchange rate, so what I ended up with was a US trade paperback which is slightly wider and shorter. This is something that Lulu needs to make more obvious, I feel. I'm waiting for a copy to arrive, so I'll let you know how it turns out since everything will have been resized.
Createspace, being default American, seems to only offer the one platter of sizes regardless of where you are.
While deciding your trim size, you may want to consider the cost of the book. Both Createspace and Lulu have a handy gadget that calculates this for you - just enter the number of pages and size and they'll provide you with a cost.
Sticking with the US store for now (since it's easier to compare costs), Lulu offers a 169 page US trade paperback (6"x9") for $3.43, and Createspace for $2.89. Lulu doesn't really offer any other options for a standard novel or short story collection, at least if you want to sell it through retailers (and you might not - more on that later). Createspace does, and this is why trim size matters. The same document formatted for 5"x8" is 227 pages and costs $3.58 on Createspace.
At first glance, Createspace would seem to be the better option, but it isn't if you live outside the US. While on Lulu you can pick your store on Createspace there isn't that option for authors, only for customers shopping on Amazon when the book is already available. If you want a hardcopy to proof, or "member copies" (i.e. not at retail price) they will only print and ship from the US. That single copy you want for proofing? It'll cost $4.88 in shipping and take up to 31 days to arrive. If you want it any sooner it's $7.99 to get that down to eight days, and $14.38 to have it in two business days. Lulu, on the other hand, offer their cheapest option at $5.10 and that's for shipping one to three days after printing. For reference, I ordered my copy on Sunday and it shipped on Tuesday.
Other POD printers are available, these just happen to be the two I've used. The best thing to do going in is to thoroughly investigate the sizes available and relative costs. Keep a copy of your manuscript open to play with the page sizes and get a rough estimate of the page count - but bear in mind this may change with the formatting.
More on formatting in my next post.