I'm 65 kg and learnt to foil and wing on a Naish crossover 120 which is 7'4, using the jet 1650 and have had an amazing time on it. Incidentally, I'm now waiting for my new smaller board to be delivered so will be selling the crossover, but my thoughts...

The water where I ride can be wild - wind against tide, waves bouncing off sea walls, wake from shipping and at times I've been thankful for all the volume. I think people are always so quick to say aim for a smaller board - there are real benefits to a bigger, longer board - stability when getting going, glide in the water to get the foil up to speed, a certain amount of inertia in the board which makes it more forgiving to clumsy footwork, and you can recover from some pretty hard touch / crash downs.

I also ride a 5'3 34 litre foil board at my local wake park, so know what a small board feels like and there no way I'd be ready to wing that just yet. It's the dream, but something to work towards.

I guess my takeaway is that foiling the 120 doesn't feel clumsy or awkward. Yes it doesn't pump as well as a small board but that's a small price to pay when learning IMHO. I'm not conscious of the size of it in use, perhaps that's because I'm using a slightly smaller foil than many. I think my progression would have been slower on a smaller board.

I've had a real dilemma deciding how small I should go for my new board - 80 or 95 litres - and I went for the 95. Reason being that the this will be my new 'big' board with my eye on the 5'3 being the windy board eventually. The ease of use for where I am trumped the extra bit of performance gain from a smaller board.

Side note - I also have the large Thrust foil which I learnt on at the wake park with the crossover. That's 1250 and known to be a 'responsive' foil - ie not the first choice for learning and I found it a challenging experience, but hey, ignorance is bliss and I use it on the 5'3 now. Point being that the foil and board package determine feel.

You've raised some controversial points - I'm not arguing this to be belligerent, I just think it's important to put across the other side of the argument: I agree with what you say about length aiding "a certain amount of inertia in the board which makes it more forgiving to clumsy footwork, and you can recover from some pretty hard touch / crash downs", BUT, I don't agree that the glide gained by length aids foiling, or that volume aids stability. To get the board up on the foil doesn't require you to get up to planing speed - the foil takes over for lift and speed way before that happens. Because of lower take off speeds (compared to normal SUP surfing) the length of a board over 7ft (possibly over 6'5) is redundant for foiling, but because it's so important for SUP surfing (where the quicker you get planing the better) people assume that the extra length is relevant for foiling as well. Length (up to a certain point) is important for fore/aft stability (i.e helps you not to bury the nose) but not the type of stability that makes you fall off (i.e sideways tilt), which is so important in choppy/unstable conditions.
Take the leaders in foil board design - Dave Kalama, Jimmy Lewis etc - their board shapes are all based around shorter, wider, thicker designs. They pack the same volume into a 5'5 board that most 7' boards have. In interviews they say that stability is determined much more by width than volume or length. Think of volume as float rather than stability. A board that is just below the surface of the water is actually slightly more stable than a board that sits just above the water because a lot of the turbulence passes over it. Having said that, volume helps you to paddle faster because if you are on top of the water there is, by definition, less board pushing through the water, therefore less drag. Brands like JP and f-one started off with long narrow foil boards, but even they are (slowly) changing to shorter, wider designs. Zane Schweitzer (one of the only people to rival Kai Lenny in downwind foil races) started foiling on a 7'6 Starboard, and has since designed a sub 5' foil board for his own use. Dave Hamasaki, (also Maui) rides a custom 4'4 Jimmy Lewis (ok, he's a small guy), Austin Kalama, Tomo - pretty much everyone on Maui rides short, wide boards with extra thickness to boost the volume - check them all out on YouTube. 'One foil boards' based in Australia - short & wide. Sunova - short & wide. Naish...ditto. See the pattern?
I completely agree with you about not wanting to go too short as a beginner - you have to be reasonably good to ride a very short board, but IMO the benefits of the longer boards (over 6'5) have all been proven to be defunct. Short boards don't take that long to get used to though. Before foiling I never went shorter than 9'7 for SUP surfing. After foiling with my 6'5 board for a few months I tried a friends 7' board and never went back.
Haha just realised I started off saying your comments were controversial, and then ended up going off onto a bit of a rant - sorry, I blame the self isolation
To put my opinions into context with my experience (as I recognise that this is all subjective and my opinion is subject to bias) - I'm 85kg's and started with a 6'5 125ltr board, now using a 5'5 105ltr board, with GoFoils maliko 280, 200 & IWA. Also regularly wakefoil with a 3'11 12ltr board. My local conditions mean I mostly do downwinding with a wing rather than a paddle. Rarely get decent waves. The sea state here is always choppy which is why i value wide boards - the very few times its been glassy smooth I could have danced on my board and not fallen off