Yes, Merchants can get lengths of 6m or more, but they have to order it in. Although, for some small sizes, they hold up to 5.4m.
For a 5m span, you'll need some fairly big sections (depending on how far the roof timbers are spaced.) Even at 400mm centres, you might need 200mm x 75mm (8x3) ........... that's just to carry their own "self-weight".
Might I suggest you have a "purlin" at the middle point of say ......... 2 8x2s bolted together to give an overall 8x4. (This is much stronger than one 8x4.)
Then you'll only need approx 2.6m lengths of 4x2 bearing on the two sides of the structure, with their other ends bearing on the purlin.