Look under “Browser type” targeting and you’ll find it just under the iPhone and Android selections
Keep in mind that mobile is a whole different game: What works on the website might not work on mobile and vice versa. I hate to beat this into your heads but make sure your offer is mobile friendly!
The 300×250’s will be the most competitive size on mobile/tablet as it’s the biggest, most clear size. Which is opposite from the website where the 728×90’s reign supreme.