I suspect that the piece of glass you are looking at is man-made. Lightning can create glass when it discharges into sand as SiO₂, the main component of sand, is the essential building block of glass. The glass deposit left over after the lightning is called a fulgerite and will tend to be very opaque and have a branched structure that mirrors the heat dissipation path from the lightning. The glass in the picture you have shared, however, is likely man-made as its edges are quite smooth and sharp and it appears to be quite clear.

The clarity of the glass is an indication of the amount of imperfections and impurities it contains; man-made glass is clear because the sand is filtered and the processing conditions tailored to have a transparent final product. In the event of lightning hitting sand on a beach, the very fast heat dissipation and impure sand (i.e. not 100% SiO₂) would result in something that would look vastly different from what you showed us. With a large enough lightning strike, however, I think it would be possible to make a piece that big; it would just look very different. I hope that helps!