Please don't feel like you or anyone else needs to do a bunch of research prior to asking a question. If that were the case there would be few questions asked or answered.

Most BG nests are about 12 - 18 inches across at the top.

Here is another bit of info on SMB beds,

From a study in the Black Bass Symp. by AFS.

SMB nested in water .5 to 3 meters where the substrate contained nest material where at least 40% of the particles were 6.4 to 149 mm in dia. ( .25 to 5.9 inches in dia.).