Re: Font-size, I would recommend keeping important text close in size to 10-12 pt Arial (since some fonts vary dramatically in size, I would do a side-by-side comparison with 10 or 12pt Arial.) I would only use 8pt for specific details.
I think most online upload sites like Vistaprint just take image uploads (.png or .jpg) but they might also be able to take .pdf; it's been a while since I've used one of them.
As far as software is concerned, I think Lucidpress and Canva both have free versions that allow you to upload up to 3 pages or something like that, which should hopefully be more than enough for the jam. They also probably have some dimensions you can snag. I believe Naomi (beatingthebinary.itch.io) used Canva to create their cards.