Use 5 and 6 m2 kites very very often, but small kites below 5 m2 a bit more rare.
Smaller than 5 m2 needed only when wind exceeds 20 knots for my average weight, but happens too of course, in conditions not suitable for surfboards but can be awesome for wave hydrofoiling or just having fun in chop
I got to ride my 3.7m yesterday in the surf At 185 lbs, this doesn't happen a lot (has to be 30+ mph for me), but it is a lot of fun when it does! I also don't change my bar - I use a 55cm bar for everything. Definitely agree with everyone else - just get used to the speed! I also don't increase my line length for my smallest kite. Generally, I only go to longer lines if there is something (like a jetty or a point) directly upwind of where I'm kiting and I know the wind will be slightly better if I have longer lines. But these are preferences, and you should experiment to see what suits your riding!
A 5m Flysurfer Boost 2 and Zeeko foil is my go to downwind foil combo when riding with my SB martes. I use lines between 20 - 36 m lines depending on wind strength. I prefer longer lines actually. Lots of fun.
I don't think I work the kite any more than normal, but might need a loop or three to get going. After that, i can just park the kite. Gybes are always downloops