Betta breeding stripes?

So I bought a female betta, but she's very small and im not sure of she'll still grow or that she's already her adult size I know they get stripes when they're ready to breed (so adult) but are they horizontal or vertical? I know one of them means they're ready to breed and the other one means they're stressed But almost all betta females I see in stores have horizontal strips? (I live in belgium, you dont see any betta stuffed in a cup here, the females are in 1 big tank and the males spread over the other tanks together with the other fish for sale so yeah) (Yeah, my betta is in a 10 gallon, cycled, filtered, heated, planted, in there for 2 weeks in quarantine before moving to the 30 gallon asian biotope tank)