A properly installed good-quality window of any size will not leak unless left open during a storm.

You will need to make sure your window is rated for a wet environment. Not all windows will function well in the shower. Your pics window supplier can pint you in the right direction there. Also, as was already said, proper installation is key. It is nice to have the light of a larger window. Something else to confer, if you are using the shower a night, even textured or frosted glass will allow those from the outside to see shadows and silhouettes.

To avoid silhouette visibility at night, you could install a shorter or higher window with the sill at shoulder height. We did that when remodeling our bathroom.

Thank you both for you input. Either way it looks like I will be replacing the window.