Hi Malvina
I have always had a different take on this subject.
Because of cost, every SUP I bought had to be a surf SUP,
Since I couldn't afford to have a dedicated flat-water board, I simply paddled what I surfed.
For 5 years, I paddled 5-6 miles on 8'10's and 9'5's, cuz that's all I had.
I loved it, I wasn't hung up on speed and loved exploring estuaries and creeks.
Smaller board are lighter and easier to transport and store.... and super fun to paddle.
It is very possible to buy one affordable board to use in surf and flat-water
Last winter, I surfed and paddle my Sunova 9'5 "the ONE".... light, well built and affordable.
Before that it was the 10'6 Naish Alana (same as the Nalu)
Before that the 9'5 Naish Mana
Before that, it was the 8'10 Fanatic Allwave
I have put together a collage of each board, with surf and flat-water shots and large (me) and small people having a blast on them.
get something you like and can afford. and start to enjoy SUP!