I have four maximas.
My smallest is about 2.5" or so and absolutely insists on remaining on the sand. Any time I've tried to get him to attach to something he jumps off. I gave up, on the sand he stays.
I have a 4" blue maxima that I was easily able to get to attach to a rock. He seems happy but I was inexperienced and didn't place him well and he's attached to the very edge so I have to have another small rock to kind of support his other side.
My two bigger maximas, a 5" aqua and a 6" black with green I originally put on the sand but they started to kind of dig down into the sand and after a couple weeks most of their shells were underground. I figured they were maybe looking for something solid to grip onto so I put a large round but kind of flat shell (I think it's a scallop shell) under each one and they attached to them quite well. This turned out to be the best as they are very securely attached and any time I need to take them out I can just reach down and grab the edge of the shell.
Hope this helps!