I curved the traveller on my old A Class, when you curve the traveller bar it's shorter accross the boat but none of the A classes travellers go right to the edge anyway. There is a good advantage to be had from curving it on just the right arc as you don't need as much sheet to let out down wind, disadvantage is clutter on the deck getting in and out over the track and if your arc is wrong the traveller won't flow out smoothly without letting of sheet. It's very hard to curve annodised track yourself with a smooth curve as it has holes in the track which create week points so be carefull (don't ask me how I know). The length from tip to tip will depend on the arc you want below your boom, with your mast as far forward as you will ever use it mark where your boom is over your decks then bring your traveller a bit inside that so your blocks are always pushing the boom toward the mast not trying to pull it away.