To set the size, under File>Page Setup, select Custom and enter 48” x 36” inches and Apply.
Rework any content that is necessary to fit and look correct.
When done, under File>Print settings and preview, check that the file looks correct.
If so, select Download as PDF and Save File. An alternative is to select File>Download>PDF document (.pdf) but there will not be the opportunity to preview before creating the file.
Open the saved file to double-check the conversion worked as expected and that the dimensions were retained. If so, view the contents at 100% to see if the quality of text, graphs, images is what you intend.