I understand that I can’t use a circular needle larger than what’s called for, but can I use one that is smaller than called for? I am wanting to make a coat for my DD and the pattern calls for a #15 32" circular needle to to finish the neck. The rest is knit on a straight 15s. What do you think? What s the rule on this as I have wondered from time to time because sometimes I see patterns say a 29" or 32" …
TIA!!
It depends on how many stitches you will be working with. On the smaller size needle more stitches will be bunched up in a tighter/smaller space versus the 32". If you are worried about dropping a stitch off of the 29" needle go with the recommended 32".
Katrina
I think 29’’ would be fine. You can use a smaller needle than required, as long as you can manage the stitches. If your stitches are too scrunched up on the needle then you could go get a 32’’, but I would give the 29’’ a try! 
I have the opposite problem, my pattern calls for a 16 inch and I’m using a 29 to ML it, and it’s too small…I need to get the 16 or go and get a 32.
However, I think for this if you’re okay with scrunching everything, then you should be fine.
I’m like everyone else it all depends on the number of stitches you need to cast on. but for the neck of a sweater 29" should be fine.
I am working on a sweater for my granddaughter and the pattern called for a 27" circular and I used a 24" and it has been no problem and I am almost done with the sweater…
Ginny