Resizing Options

We do event photos and we don't offer any downloads as we email all sales.  I'm looking to save on website file storage as we can easily shoot 250,000+ photos during our 10-day event, which is why we can't afford to keep all the higher-res online for downloading.  Is there an optimal photo-size setting I can use for my specific needs, since we don't allow downloads? 

All I really need is the thumbnail and a pic of 700 pixels wide (max) which the client can zoom into, in the individual photo screen.   

My old photocart  bhriders.sturgis.com
New Sytist (in progress)  photos.sturgis.com    

a 700 pixel image will look horrible zoomed in on. But just upload that size, the sytist zoom-in i believe is hard coded and can not be changed and only works when a upload is big enough.

I would suggest around 1600 pixels. It won't be that large of a file and will have some zoom.

OK, thanks guys.  I'll look to do this.   :)