There’s a tab in the project window that has the format setting of your project. I forget what it’s called, I think it’s left of the hardware tab. Click that, change it to 25i, now go back to transcode, and you should have SD options.
Was there any still photography taken on set that could be used as an alternative? If the still from the miniDV degrades when blown up for print, there are some software packages you may want to consider – see https://photoenlargement.imagener.com/ for one example. I haven’t used this particular tool myself, but a guy I used to work with was a fan.
If your original is a miniDV master, then your image is 720×480 pixels (720×576 if it’s PAL) regardless of the system you use to import it – but do be sure to capture at 1:1 resolution. DPI doesn’t really refer to a video image, but rather a printed one. At 72 dpi your image will be about 10×6 inches. ~Double the dpi to 150 and that gets cut to 5×3 inches.