I don't disagree with the idea of going as long as you can but generally with the long slack geometry, the rear end is quite short. I haven't done anything that didn't comfortably fit on 1220mm long beam. Doing some rough calculations, the wheelbase would need to be upwards of 1300mm before 1220 wouldn't be enough. The front of the beam sits roughly inline with the HT bottom edge, not the axle. I know another 5-6inches of length on my beam would be annoying to as it would make it that much harder to get around it in the shop.
Here's an example of a long slack and if you're building a fixture based on the Arctos design, some of the front center is excluded. I'm not saying the below is perfectly measured out but by my calcs, even the below would fit on a 1220 with 200mm (8in) of extra length.
Anyway, if you have the room, why not have the extra length - you can add hangers for common fixture tools and what have you. I don't think you'll be limited by going with 1220 or even 1000mm and that saves a fair bit of shop space. Post up pictures as you go!
