You can create your own diagnosis function that can be checked to run when executing the diagnosis tool. 

Diagnosis has 3 apis to export data. Each may have different color on the diagnosis result screen. 

k.diagnosis.error

For critical error message. 

k.diagnosis.warning

For non-critical errors. 

k.diagnosis.info

For information only.

 Create a diagnosis, name it "mydiagnosis", and enter below to the body

 var allpages = k.siteDb.pages.all();   
 var page = allpages[0]; 
 if (page.body.length> 20000)
 {
    var error = "Page too long: " + page.name;  
     k.diagnosis.error(error);
 }

Go to the diagnosis tool on main navigation menu, you can find the diagnosis option now.