Hi Han,
There are several solutions:
1. Download a template, and change its size. If it is latex, try to find the command setting the size and edit it. Or if powerpoint, you can change the page size from somewhere. I don't have a windows OS at hand now to find the exact the place, but I remember I did that before.
2. You can prepare a poster with any size, but print it with size smaller than 32*30. To do that, just mention the size in the email to the SCS poster print office, then they will call you to ask more details if they don't understand your requirement.
3. It is fine to bring a poster whose size is not exactly equals to 32*30, but not too small or too large.
Thanks,
Mu