I'm sure this can be done, just cant figure out how. I'm needing to export my portfolio as a pdf but my indesign document is a3, i see no option to chose the size.
A workaround is to take your A3 indesign document and place it inside an A4 document.
Alternatively, you can export the A3 document as an A3 PDF and insert this at stage 3.
You can't change the size of a document on export, you'll need to change the document size and then export it.
Choose: File->Document Setup (Command+Alt+P) and change the size there.
Then you'll be able to export it as a PDF.
Open a new A4 document. In the A3 document "Select All" and "Copy" Go to the A4 and "Paste" Then incrementally re size all of the elements together until you get the desired size.
You can press Page tool (Shift+P), which is normally the third icon under the white arrow into the Tool Bar. Make sure that all the layers into the document are unlocked, then in the Liquid Page Rule press Scale, then press Ctrl+A to select the pages and give the size of A4. It automatically scale the content to A4.