RESA is not part of the declared distances for performance purposes. It's an aerodrome requirement to provide them so in the event of an overrun/undershoot the aircraft has some extra safety margins. Minimum required length of RESA for most runways is 90m but recommended is 240m.
If a stopway is provided then the RESA origin starts 60m (end of runway strip) beyond the end of stopway. If stopway is not provided then the ASDA/TORA will be the same and RESA origin starts 60m beyond.
RESA width has to be at least twice runway width. Overrun/undershoot RESA lengths vary depending on position of end of TORA/ASDA and the start/end of LDA.