Export to PDF and open that in Photoshop. You’ll have way more flexibility.
If this is for a print publication, then convince your client to go with the PDF file. Create it with a PDF/X-1a PDF preset so it will be acceptable wherever the ad is run.
JPEG will have the limitation of size. It will need to be 300 ppi if there are any type or vector elements and Maximum Quality. You could choose lower quality (more compression) but then any type or vector elements will lose their crispness.
thanks for the quick responses - I was worried that the export>pdf>photoshop>jpg was the best jpging solution. Are there still screen based platforms that can't take pdf's? Are there really screen based instances were you can only use jpgs? I am way more print based that screen, so I feel I am in catch up mode.