Surfing Waves

Hello, I am about to buy my first own board and I am an intermediate surfer with all together like 4 months of surfing experience. I am 164cm and 60kg (5'3" and 132 lbs, I looked it up ust for you guys:))
So I wanted to buy a fish but a bigger one, like 6'6'' or 6'4'', which would be the right size for me, coz I can surf a minimal 6,8 no problem, and I wanted to have some challenge with my new board. But at the shop they tell me that for my size, if I want a short board, I would need a 5,10 or smth. Because 6,4 would be a board for smb bigger. Surely 5,10 would be too small for me, I know I cannot manage it yet.
So I am confused.... can anyone help?

I am 5'3" 125 lbs. and own a NSP 6'4" Fish. It is 20 7/8" by 2 9/16. Being that is molded epoxy, it is very floaty.......maybe too floaty, since I can catch waves out by the longboarders. But I can also duck dive it, so maybe you'll be able to with some practice. If you can easily surf a 6'8" Funboard and turn going backside, I would go with 6'0"-6'2" Fish. It kinda depends what the board is made of and other dimensions, NOT just length. But I definitely say you can go shorter than 6'4".

To give a brief reply, I think that a 6'4 may be to large especially if you consider yourself an intermediate surfer for your weight you may find it quite floaty and may find it hard to turn... I imagine your going down in size for sharper turning? To give reference I am a 6'2 guy weighing 85 kilos and I ride a 6'6 fish and it's pretty spot on bear in mind that fish boards are far more boyant (usually) than a normal short board... My advice take a 6'2 out!