just looking at a wind forecast vs actual you can see a huge variation. what do people normally choose? Size for the gusts or lulls or average? You wouldn't know before hand but can guess based on past variation in a particular spot time of year? So if its forecast to be 20kts I'd go with 11m kite (13-29 knot claimed range)

Rig for the average wind but make sure the kite can be trimmed and handle biggest gust if the day

For 13 to 29 I would use my 10 if average wind was 20 knots. But if ave wind was 25 with occasional lulls I might run my 8m.

If the wind was blowing from 13 to 29 knots it would be a crap session - seriously just not worth going out.
No kite can comfortable cover that wind range and Im of the opinion that any more variation than 10 knots means you will be seriosly compromising one or both ends of the wind range ? Im not into kiting stupidly overpowered so Im sure there will be kiters on here that will call me soft - every one to their own ! Beginner to intermediate kiters should never go out when the gust are more than double the lulls unless they want a position in the Darwin Awards!!

I'm with lambie, I love a storm kite but it has to be nuking, or let the main front pass and kite it when it's settled down, also it's the spikes you get, they're the dangerous ones not the gradual ones,

Here's what we had the other day constantly through the morning, a 7 could handle it but it would of been Russian roulette , the sky never changed to show any inclination of a frontal 20 knot gusts no matter what size isn't worth it, there's a video of Lenten and langaree chasing big storms, it's the worst I've seen them perform on a kite

one of the things about winter v summer where i live . 13 or 8 ? winter is always windier before rain and lulls after . 20 to 40 knots is noy that much fun.
weather, and knowing where you are in relation to weather fronts is really the key .if youre going out before a change , i have found always better to err on the side of caution. sometimes under powered but always in control
i find summer breezes a lot more predictable .
new kites are way more forgiving than earlier ,(on my 11th kite) latest fone has huge sweet spot with range , i am sure most new kite developements would be similar,

Don't rig too big until you've had plenty of wallopings in 15 to 20 knots. If you get walloped in 30-40 knots you could easily die.

The old windsurfing rule applies equally well to kiteboarding. Choose your sail for the gusts and your board for the lulls.

You can get away being a lot more overpowered with a kite than a sail, but the basic rule still works.