Currently the VM “Size” blade is greyed out for your Azure Spot VM is because resizing is currently disabled for Azure Spot.
Thus you cannot resize your Azure Spot VM to another size, due to the current limitation of Azure Spot VM the only solution to this is to continue to use the already chosen VM size , when it’s capacity is available in the Azure spot cluster. Or you can delete and try to create a new vm
Please note that Azure Spot VMs are only recommended for workloads that can handle interruptions like batch processing jobs, dev/test environments, large compute workloads, and more.
Also note that the spot instance are currently in public preview and this versions are not recommended for production workloads, as certain features might not be supported or might have constrained capabilities.