As a traitor in TTT, you can see a timer and a distance number (probably hammer units, but I'm not actually sure what the number represents) for any armed C4 explosives. How far away do you need to be from the bomb to survive?
according to this Steam discussion the radius increases when the timer gets increased
Traitor Tip: The C4's blast radius depends on its fuse length! If you're in a map with two other innocents, lure them to your one-two minute bomb.
I've found on a 45second c4 the minimum safe distance with out taking damage is about 850 units (assuming the server you are on displays those, this information is based on the c4 of DinkleBerg's TTT).
Not to Necro this thread but I feel since it's on a top google result regarding TTT c4's it's nice to have a more definitive answer for anyone else curiously searching!