If you want to encode repeating groups larger than a uint16 then you need to specify an increase in the size of the block length property:

<composite name="groupSizeEncoding" description="Repeating group dimensions" >
    <type name="blockLength" primitiveType="uint32"/>
    <type name="numInGroup" primitiveType="uint16" semanticType="NumInGroup"/>
</composite>

But if you do this then SBE will generate Java code with a compilation error in:

RedactedDecoder.java:121: error: incompatible types: possible lossy conversion from long to int
            blockLength = dimensions.blockLength();

I can understand if you don't wish to support block lengths larger than a uint16 but I think doing this with SBE validation rather than generated invalid Java code would be a better approach.