50x the power takes 50x the surface area and 7.1x the length / width. Or less than that with a lifting body approach. Or significantly less than that if it's daytime only.
Solar Impulse is a marvel of engineering, and the fact that during on of the legs of the flight a single person flew the damn thing for 6 days straight is an attestment to human ingenuity and courage but there is a very very big difference between flying 50 people using electric aircraft (again electric not specifically solar) in 50 years to flying 50,000 of lbs on solar power.
We already have electric aircraft capable of 1.5-2h of flight scaling those up on electric power alone without solar might actually be easier as you do not inherit the weight of the solar panels. Efficient fuel cells and ultra light batteries / nano super caps could potentially make electric flight possible within 50 years but again relying on solar to transport cargo long haul no sorry not happening.
You would probably want a thicker coating to protect the cell, but in 50 years solar can add very minimal weight.
Second wing area and lift is related to velocity (not power) right before you stall outside of that it's a looser connection. Velocity is indirectly related to power in steady level flight based on drag. Drag in level flight is a function of weight, shape, altitude, and velocity.
Economicly I don't think this is a good idea. Physically it seems possible with cheaper and more efficemt solar cells it might make this more reasonable up to a point. Especially if they can boost altitude and thus speed.