There are 2 things you should look at for this.
1. Websites statistics resolution stats – What resolution do most of the people that visit your website already use? Use these stats to determine what size you should do it in.
2. Internet trends – Check the current resolution stats, although keep in mind that this is the entire internet and your particular website might have very specific trends. Again use these 2 pieces of information together, i.e. the trend of the internet users and your specific website.