This is defiantly a sail maker issue. On most boats I would very heavily recommend against a 120 though. On some boats it might work, but for most it will be too large to go inside the stays, and too large to have a decent trim angle.
For most boats the best option is to go as large as possible inside (typically around a 105), or go large enough that the angle between the foot and the clew stat to come back to reasonable (135is). But for most boats the payoff is pretty bad until you got to the 155 range.
Typically a well cut sail will only need about 18" of track to cover most wind
conditions the sail was designed for, so it doesn't need much. However a longer track does allow you to use the same cars with different sails
(at some loss of performance since the lead angle isn't optimal for each sail).