I had the same problem with this. It seems that the custom paper field is interpreted as inches by the print preview, but interpreted as millimeters by the pdf export function.
If I enter 457 x 610 in the mm fields (equivalent to 18" x 24") then I get an enormous sheet size in the print preview, equal to 457" x 610".
If I enter 18 x 24 into the mm fields, then I get the right preview print size, but the PDF export yields an 18mm x 24mm paper size, so it display only the lower left corner of the previewed sheet.
I ended up adding 18" x 24" paper edge lines around my border/title block and exporting it on B2 paper. I took the files to Kinkos/FEDEX and had them crop the image along those lines, then print on 18" x 24".
If they get this issue and the crashing when saving issue fixed, I think the program represents a fantastic value for the occasional user. I have certainly experienced many frustrations with lost work and the exporting issues, but I did finally finish a simple 4-sheet drawing set. The final product was fine.
Looking forward to the next version...