Front derailleur spec depends upon a number of things:
1) What shifter are you using?
2) What crank are you using? If using an older boutique crank with a longer bottom bracket axle you may need a derailleur that has more travel to the outside towards the large chainring.
3) Seat tube diameter size which someone else noted above as 31.8mm which is correct for 98)
4) Cable routing on the bike
By 98 many had gone to bottom pull / top swing which was suppose to have less flex and allow for quicker up shifts to the large chainring. I never cared for top swings because I think the had more of a tendency to throw the chain of on the inside when downshifting.
Needed: 26.8mm XTR seatpost, blue GT/Grundig Jersey.