You can gain a maximum of 10m on each end of the 2nd or 3rd legs (but not both), if you push the exchange location and risk getting disqualified by not making the exchange. By the same measure, either the 2nd or 3rd legs have the potential to be the shortest as well. For the 1st and 4th legs, the length can only change by 10m.
1st: 90m to 110m
2nd: 80m to 120m
3rd: 80m to 120m
4th: 90m to 110m
Realistically, if you try to gain/lose more than 5m at any exchange zone, you risk running into problems getting the baton around. It’s much safer to extend the length of a relay leg at the start of it, rather than at the end of it, where, if you miss the pass the first time, you probably won’t get a second chance.