are the dimensions shown (particularly the length) the overall size while bag is open or the interior useable size when the bag is closed and clipped?

Tech Specs:

Material:

PVC-free Wxtex 

Closure:

Roll-top 

Volume:

[S] 488 cu in (8L); [M] 1220cu in (20L); [L] 1891cu in (31L) 

Dimensions:

[S] 12.25 x 30.75in (31 x 78cm); [M] 16 x 44in (40 x 112cm); [L] 16 x 57in (40 x 145cm) 

Backpack Straps:

No 

Weight:

[S] 13oz (368g), [M] 18oz (510g), [L] 23oz (652g) 

Recommended Use:

Sea kayaking, kayak touring 

Manufacturer Warranty:

Lifetime