1. Your Weight
2. Skill and Progression Speed
3. Local Winds

Weight
Your weight is the largest factor for determining wingboard size for starting out. Once you have skills; however, your weight can be almost completely disconnected to board sizes especially in high wind areas.

Skill and Progression Speed
Your skill and progression speed is another factor that will affect your wingboard size. If you have years of windsurfing or SUP surfing experience you will progress quickly in wingfoil. This means you can probably start with a board a little smaller than what is recommended for your weight. Board balance will be better and therefore you will be able to handle small board with less displacement.

Riding Conditions
Your riding conditions will have a large impact on your board size as well. If your winds are mostly light you will want a larger board in general because you will be spending more time floating around working to get up on foil. If you're in a windy area large boards can be problematic as they blow around etc. You still will need to start standing on a board so you'll need enough displacement to support your weight even in high wind areas but you will progress more quickly to a small board in these areas. Light winds are considered an average of 10-18mph and high winds are an average of 18mph+.

Weight 100-130lbs
normal progression/light winds 85-95 Liters 
fast progression/light winds 65 - 90 Liters 
normal progression/high winds 65 - 90 Liters 
fast progression/high winds 55 - 75 Liters 

Weight 130-160lbs
fast progression/light winds 65 - 95 Liters 
normal progression/light winds 90 - 100 Liters 
normal progression/high winds 65 - 95 Liters 
fast progression/high winds 65 - 80 Liters 

Weight 160-190lbs
normal progression/light winds 100-110 Liters 
fast progression/light winds 75-105 Liters 
normal progression/high winds 75-100 Liters 
fast progression/high winds 65-85 Liters 

Weight 190-220lbs
normal progression/light winds 110-120 Liters 
fast progression/light winds 88-115 Liters 
normal progression/high winds 85-110 Liters 
fast progression/high winds 75-95 Liters 

Weight 220lbs+
normal progression/light winds 130 Liters + 
fast progression/light winds 120 Liters + 
normal progression/high winds 99 - 120 Liters 
fast progression/high winds 85 - 100 Liters 

Inflatable Wing Boards
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Slingshot Tracker
F-One Rocket Air
Fanatic Sky Air Premium

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North Seek 
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