The potting medium could be peat or compost or a mixture of peat or compost with sand or other ingredients. Although some will tolerate having wet soil, most do not. Avoid water with high salt content, especially sodium, as this will burn the plants and damage the very fine roots. Bright light would damage most begonias, though it does not burn many cultivars of wax begonia. The aroids respond to fertilizer.
Large leaves are the result of ample fertilizer.
A fertilizer higher in nitrogen than phosphorus and potassium is common. The roots of begonias are especially sensitive to salts. Careful application of inorganic fertilizer is essential. Most thrive with fertilizers containing a little more nitrogen than phosphorus or potassium, but those grown for heavy displays of flowers need a fertilizer with adequate amounts of nitrogen, phosphorus and potassium. Since begonias are grown in media high in organic matter, their nutrients can be derived from organic matter.