Small: 4 inch standard nursery pot.
Medium: 6 inch standard nursery pot.
Grow in a spot with bright light (enough that it casts a medium to strong shadow most of the day). If it does not get enough light, it will grow tall and spindly, with weak stems and no flowers. In Northern areas, it is usually able to take some direct sun through a window, especially in the winter. In more Southerly areas, it may suffer sunburn if it gets a lot of direct afternoon light.
Water when the top few inches of the potting mix start to dry to the touch. It is rather susceptible to root rot if overwatered (especially if it doesn't get enough light). If it stays too dry, your polka dot begonia may develop brown, crispy leaf tips or edges. It may even drop new leaves as they develop.
Plant Specific Care
Native to South America, polka dot begonia appreciates above-average relative humidity levels, but typically tolerates average indoor humidity. If the air in your home or office is dry, grouping your polka dot begonia with other houseplants may help. You can also place a small humidifier nearby or grow it on a large pebble tray filled with water (so that the pot sits on top of the pebbles, above the water).
You can fertilize your polka dot begonia in spring and summer if you choose for more/faster growth. Any general-purpose fertilizer labelled for use on houseplants will do. Just be sure not to exceed the directions on the product's packaging.
Pruning usually isn't necessary. If your plant gets too tall and lanky, with no leaves at the base, pinch it back to promote more bushiness and branching. This creates a fuller-looking plant.