After years of windsurfing, winging, SUPsurfing i (56 yo / 95 kg) want to take the challenge of supfoiling in (small) waves and downwind in small conditions and even flatwater paddleups.
In winging i usually use frontwings of 1050 and 850, so i'm used to small frontwings (for winging).
For the "new for me" disciplines so far i use Sabfoil T8 based frontwings:
Paddleup: Leviathan 1750 / 2400 sqcm (going as big as possible to learn)
Supfoil/Downwind: : Tortuga W1250 / 2400 sqcm (high lift at low speeds)

But looking at all the vids and the good supfoil/downwind guys at the spots they are on much smaller frontwings 900 - 1200 cm?.
It seems nobody is using a wing like the Tortuga W1250 anymore, but looking at the vids and discussions form 3-4 years ago e. g. the big GoFoil wings GL210/240 or even Maliko 280 where the frontwings to have.

I wonder if it's still a good choice for a heavy beginner like me to use a big frontwing like the Sabfoil Tortuga 1250 for Supfoil/Downwind or if i'd better look out for smaller HA frontwings (from Sabfoil, eg. Leviathen PRO, Razor etc.) to start learning?

Go with a big front wing into little bumps and waves. Do not attempt to DW unless you can get up in little bumps and ride swell if possible.