You can specify the VM size for nodes but not for the master nodes.
You can use az aks create with --node-vm-size argument to do this. Please check the details here.
Please note that System node pool must use VM sku with more than 2 cores and 4GB memory.
az aks create --resource-group myRG --name testakscluster --node-count 1 --node-vm-size Standard_DS3_v2
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