it looks to be roughly 19–20 inches (about 48–50 cm) across, which is a substantial specimen. The characteristic deep, wavy ribs on the outer shell and the smooth, pearlescent interior are classic Tridacna gigas features.
A few notable things about what you’ve got:
∙ Giant clams can live over 100 years and reach up to 4 feet (1.2m) in the wild — yours is a very respectable size
∙ The pitted, weathered exterior suggests it’s been dead for some time, likely bleached and dried naturally
∙ The inner valve (Image 1) still shows beautiful iridescent layering