Maximum size sediment

What is the maximum size sediment that should bed used to define sediment classes or the sediment cores in EFDC. I have recently tried to use sediment cores which include a gravel class between 2 to 64 mm. However, when interpolating the core files onto the bed it appeared to remove the gravel class, as the interpolated D50 values were all within the sand class range (even though the D50 of the cores were in the gravel range). So I am wondering what the maximum sediment size is that should be used for creating the sediment bed.

Thank you