How to choose circular needle length for a project?

So I’m a self-taught knitter, and I’ve recently ventured into knitting in the round - tried making a scarf that ended up being more like a hula hoop XD anyway, now I’d like knitting in the round using an actual pattern that I picked up from Hobby Lobby for a pillow. It says to use size 25 mm round needles, but doesn’t specify the length of round needles to use. It does say that the final product will be 18" x 18", so I was thinking that I would needles shorter than 18" based on my limited knowledge, but I can’t seem to find any size 25 mm of that length. The project only has you cast on only 13 stitches, so I really don’t think that the smallest length I can find, which is around 30", will work. Any help is appreciated!

What’s the name of the pattern? Can you post a link here please. Someone else may be able to answer based on the info given but I’m really confused. Are you to work in the round with 13 stitches or are you working flat on circular needles? I can’t quite see how either would be 18".

I did a quick look on Ravelry and it seems you should get about 1 st/in.

If you have double points of 25 mm, use those instead if you are ued to them. Yes, a circular needle is easier and there’s nothing to drop/fall out/lose and dps will work just as well.

Is the whole scarf 13 sts or is there an increase?

And yes, is it worked in the round or flat??