This is the 309th most frequent French word.
petite
Small; little.
Here, 'petite' is used as an adjective modifying the noun 'maison' to describe its size as small.
Elle a une petite maison au bord de la rivière.
She has a small house by the river.
In this sentence, 'petite' functions as a descriptive adjective to indicate the stature or height of 'ma sœur'.
Ma sœur est petite, mais elle court très vite.
My sister is short, but she runs very fast.
'Petite' here is used to characterize the size of the 'carte' as small or little.