When you are at the comments menu in wp-admin you only see 20 comments at the time, so if you want to delete alla comments you can ONLY mark the 20 comments you see on current page. It dosent mark comments that are on the next page.
Found a solution but dont know where to look for it
In WP-admin -> Screen layout there you can see that its only set to 20 comments a page. I know that i could just access the screen layout and set it to maximum (999 comments a page).
Now the question is can I set that value (999) to default for every user i create ? Or just basicly for the whole network?
Follow question:: If its possible, then whats the best way to do it ?
NOTE : Im using latest WordPress version with Multisite network.