How to change print size in Spark? Designs are being saved as 8.5 x 11...

Greetings! I have a print customer who is using Adobe Spark to create two files that are a full bleed custom size. one is 18" x 11" and the other, 10" x 11" - she can set the size and create the files, but when she goes to save, the resulting files are 8.5 x 11. I watched her do it just now; when she opened the files in Acrobat Reader, the print size on each was 8.5 x 11, with the images shrunk to fit so the proportions were correct (not including the surrounding white space of course.) Both files use a large amount of text so cropping and sizing to fit on our end is not an option - much of the text is sub-standard.

I'm not familiar with Spark enough to know what to look for to change the actual print size of files created with it; can someone out there provide a bit of information I can pass on to my customer?

Thanks very much!