As Sonicbum mentions in high moisture areas the entire radar screen can be red. A TS isn’t 40 nm by 40 nm but that’s what the radar display will show.
The newer radars that allow you to desensitize the radar returns helps.
If if you don’t have a newer radar, that allows desensitization, remember the overall radar image at top of descend, or lower, before the entire display goes red.
Years ago descending into a tropical storm I had a senior CKA/training manager on the Jumpseat. I mentioned ‘red out.’ He’d never heard of it (they don’t fly as much as regular line pilots) and was skeptical. At altitude it was obvious, both with the naked eye and radar, that a big cell was over the final waypoint of the STAR. After descending into the heavy moisture the TS return was lost as the entire screen turned red. We deviated around the known weather based on its location from the radar display before it went entirely red.
Today it might be worth using your camera to take a picture of the radar display. Fortunately the vast majority of our fleet has the improved radars that allow desensitization.
Phone cameras as can also be used to view a dim radar display in bright light. Put you camera lens so that it displays the ND or particularly a radar return. It greatly increases the brightness of the ND display.