When learning to foil, it is recommended that you have plenty of power in your kite -- so most people recommend the same kite you'd use to have adequate power on twintip (not boosting to the moon, just able to plane easily). That way you have power on demand for learning the waterstart, can plane with foilboard on surface, then eventually get up on foil. With this kite sizing, you should find that bodydragging is same or easier on a foil. You rest your body kind of half over the board, point the foil upwind, lean into it, and you'll go quite fast. Possibly even feel foil trying to rise!
But once you get better at foiling, you'll find the apparent wind you generate means that you get over-powered quickly and you will enjoy a much smaller kite. I now find I prefer 2-4m less in size than what others at my beach are using with twintips. EG 8m at 16-18kt, vs normally at 25-30kt.
However, the challenge this brings is that our spots require bodydragging offshore to get through the kelp beds. If I am perfectly comfortable foiling my 8m in 18kts, this may not be enough wind for easy consistent pull when body dragging. You have to work the kite more, but that's challenging one handed, with other hand on board. I find I have to angle it more downwind than I prefer, to get enough speed to not bog down. So yes, it's a challenge!
A similar problem is in super light wind. You have enough wind to fly the kite and enough wind for foiling once you're up and riding. But getting between the launch and up and riding can be pretty iffy. It is especially challenging in onshore winds, as you can wade out to chest deep, but you can't afford any downwind movement in our waterstart, but you don't have enough power to start fully side to the wind. And not enough power to bodydrag. Those are the days where you honestly may be better off doing something else ... unless you are able to get it working, then you're the champ for the day, maybe the only person on the beach able to ride while everyone else watches.