If your house was designed with passive solar heating in mind, the rule of thumb is to keep the glass ratio to floor area no more than 6 - 7%.
You also want to choose a window with a high Solar Heat Gain Coefficient (SHGC) and a low U-Value.
For more information:
Read "What's the best way to capture solar heat from south-facing windows?" a Q&A answered by Cassandra Adams.