The standard size for shower curtains is 72x72 inches. This will work in most bathrooms unless your shower was custom-made.
If that's the case, measure its height (from the rail to the floor) and keep this in mind:
Your shower curtains should come around 12 inches down from the side of your bathtub in order to avoid a wet mess;
However, they shouldn't actually touch the floor! Aim for at least 2 inches between their hem and the bathroom floor: shorter curtains result in a more casual look, whereas longer ones are more elegant.