Sizing pop up photos

My goal is to post photos for clients to download in their original high quality (1800px by 1200px). When the images pop up in the pop up box, they show up full size and I cannot see the whole picture because the picture is so large.
Is there any way that the picture size on the pop up could be restricted to a particular size or percentage maybe 600px by 400px or 40% of original? Otherwise, I will have to post lower quality pictures which will not make my clients as happy.

Thank you for your help

Sorry this all was in English.

~Phil M