Use large Grade A eggs for baking. Substituting another size will cause trouble. Size variations with eggs may not be apparent to the eye, but the difference in volume between six large eggs and six jumbo (or small) is enormous.
Eggs brought to room temperature will beat well because they incorporate the most air. On the other hand, chilled eggs separate most easily. (In recipes in which the eggs will be heated over hot water, as in Biskuittorten [sponge cakes], the temperature is immaterial, so use chilled eggs if you wish.) For room-temperature whites, separate the cold eggs, opening the eggs over a heatproof bowl so the whites can fall into it. Place the bowl in a larger bowl of hot tap water and let stand, stirring occasionally, until they lose their chill, about 2 minutes.