Determining the largest high school in Wisconsin definitively is difficult because different metrics (enrollment, building size, etc.) can yield different results, and data fluctuates yearly. There's no single, publicly accessible, consistently updated database ranking Wisconsin high schools by size.
To find this information, you'd likely need to contact the Wisconsin Department of Public Instruction directly or individually research enrollment numbers for the largest high schools in the state's major cities (like Milwaukee, Madison, Green Bay, etc.).