I have a large and very good Lowe Alpine rucksack that can be borrowed. With the extending lid, it goes up to 90 litres, about 75 with the lid in normal position.
I don't know the north coast stretch, but the spectacular stuff is on the South IMHO. Make sure to pop inland at Bosherston by way of the lily ponds, as (a) it is gorgeous and (b) there is a pub there. The stretch by St Govan's has the chapel, the Castlemartin tank ranges (which can divert the path), the Green Bridge, Huntsman's Leap and an awful lot of seabirds. Much auking and a lot of choughs. Views out to Lundy.
Bosherston also usually has a lagoon , so that those fond of al fresco bathing can find a little warmer water. There is a campsite inland at St Petrox (don't know if it is still open) that has an old church tower one can ascend for ace views. Seaside cafe used to be by Traeth Mawr, campsite there now, and the ascent afterwards used to go past a mini 'Pentre Ifan', regularly used in dramatic photos but which is only about two feet high. Not to be confused with Coetan Arthur.