There is not a sizing difference to bolt to the engine itself, but the 94 gsr header is physically shorter then an ls header (front to back) and it doesn't have a place for the o2 sensor.
That is why it is sometimes harder to find a header for a gsr on ebay for example.
I would say if it says the header is for a GSR only, then it will fit your car, if it says it will fit all models integra then I would be worried.