Can I Expand a Single Shard of a Cluster Instance (Scale-Up)?

No. You can only add more shards to expand the instance capacity.

If you want to use a larger size in each shard of a Proxy Cluster instance, change the instance to the master/standby type, and then change it back to Proxy Cluster with the desired shard size. Before changing the instance type to Proxy Cluster, evaluate the impact on services. For details, see Notes and Procedure for Enabling Multi-DB for Proxy Cluster Instances