You can use the following callbacks (exported by madExcept.pas) to be notified about when the window is created and then at that point in time you can probably simply increase the window size:
// do you want to be notified when a madExcept exception box was created/destroyed?
var
OnExceptBoxCreate : procedure (exceptBox: HWND; simpleMsgBox: boolean) = nil;
OnExceptBoxDestroy : procedure = nil;