Thanks for reaching out. Book fonts, text sizing, and spacing vary a lot by genre and personal preference of the authors, so there isn't exactly a universal standard.
I would say you can't really go wrong anywhere between 10 - 12 font size, and spacing usually runs around 1.2 to 1.4 for the majority of books, but you can always do some Googling to see if you can find a standard for the genre you're planning on publishing.
You can also find a lot of resources on formatting at kindlepreneur.com.