If you are replacing the headset, the pertinent measurements are stack height, thread spec, and fork crown race seat diameter. A 1990s Raleigh almost certainly has an English thread (1" x 24tpi) steer tube. Depending on when and where the frame was made, it may have a JIS (27.0mm) or ISO (26.4mm) fork crown race seat. The only way to be sure is to remove the crown race and measure the race seat. Stack height is a measure of the space taken by the various headset components. Your replacement headset should either be the same or shorter than the original headset. The difference in distance between the top and bottom edges of the head tube, and distance from the fork crown race seat and very top of the threaded steer tube will tell you the maximum stack height your frame/fork will accept. Typical measurements of this distance will be 30mm to 40mm. The shorter this difference, the more limited your selection of replacement headsets.