I'm 90kg wet and have been SUP surfing a 8'5" Naish Mana GT for several years, I bought this "smaller" performance board because I didn't want to buy the standard 10 foot barge and then sell it when I got the hang of SUP surfing after years of short board surfing.
when I first got that board I struggled to stay on it in the surf and got bounced off by nearly every bit of chop, backwash or just fell off paddling for a wave. I got really frustrated and thought I'd made the wrong decision
then I just went and did a heap of paddling on that board in flat water (lake) and that confirmed that I could paddle the board and not fall off. Did tail turn drills, J stroke paddles etc
I then went back in the surf and Wallah! no problem staying upright in waves, chop etc and progressed really fast from there
Recently started the Foil journey and thought SUP would be the easiest transition as after several years of SUP surfing I'd be sweet

Ended up with a used 6'6" Blueplanet EasyFoiler around 112 litres and found that I fell off all the time in the surf and most of my energy was used in climbing back on the board after every fall, not talking about actual foiling, just trying to stay upright paddling around waiting for suitable waves.
So I went did the flat water paddle practice thing and worked out to stand pretty much over the centreline of the board and more bent knees plus getting used to the slow side to side rocking motion of having mast and foil underwater and that your reactions need to adjust to the delay. So back into the surf and didn't fall off quite as much but still not as comfortable as paddling a regular SUP. Then that board got a massive delamination bubble and was going to cost big $$$$ to fix, I was really bummed and pissed off that I had to spend more money on gear (ha, ha.)
Anyway, at that time the E3 was just released and something about that board clicked with me so I was able to snaffle a 5'10" 123l E3 SUP
Dyl from Kalama Australia suggested if you go shorter don't go less volume at the same time
I can stand and paddle the E3 absolutely fine on flat water and can stay upright in the surf if its not choppy. Paddling for waves can be tricky, really need the J stroke dialled to be in the right position and angle
SUP paddling a 5'10" board is quite a challenge but very satisfying when you get it right. I also use the 5'10" for prone foiling, wing foiling and flat water SUP cruising with just a mast (no foil) so it doesn't just go round in circles
Moral of my little story, start bigger and easier and get the basics nailed. If you try and learn on inappropriate gear you will get extremely frustrated and your progress will be slow or none at all, increasing your frustration and "non froth"levels

I've recently bought a very cheap 5'2" prone board to try and it's been a revelation. I've progressed very quickly on the prone and feel that I should have learnt prone before SUP, but that's another long winded post