I want the script to list the storage account container details and storage account size from all the subscriptions in my tenant filtered out by tagkey.

You can use the below script to get the list of storage accounts with usage, container name and filtered out by tag key.

Script:

Connect-AzAccount -Tenant 'xxxxx' -UseDeviceAuthentication
# Get all subscriptions
$subscriptions = Get-AzSubscription

# Define the tag key to filter by
$tagKey = "createdby"

# Initialize an array to store the results
$results = @()

foreach ($subscription in $subscriptions) {
    # Set the current subscription context
    Set-AzContext -SubscriptionId $subscription.Id

    # Get all storage accounts in the current subscription
    $storageAccounts = Get-AzStorageAccount | Where-Object { $_.Tags.ContainsKey($tagKey) }

    foreach ($storageAccount in $storageAccounts) {
        # Get the metrics for the storage account
        $resourceId = "/subscriptions/$($subscription.Id)/resourceGroups/$($storageAccount.ResourceGroupName)/providers/Microsoft.Storage/storageAccounts/$($storageAccount.StorageAccountName)"
        $uri = "https://management.azure.com$resourceId/providers/Microsoft.Insights/metrics?api-version=2023-10-01&metricnames=UsedCapacity&aggregation=Average"

        try {
            $response = Invoke-AzRestMethod -Method Get -Uri $uri
            $metrics = $response.Content | ConvertFrom-Json
            $usedCapacityMetric = $metrics.value | Where-Object { $_.name.value -eq "UsedCapacity" }

            if ($usedCapacityMetric) {
                $averageCapacity = $usedCapacityMetric.timeseries.data.average | Measure-Object -Sum | Select-Object -ExpandProperty Sum
            } else {
                $averageCapacity = 0
            }
        } catch {
            Write-Warning "Failed to retrieve metrics for storage account: $($storageAccount.StorageAccountName)"
            $averageCapacity = 0
        }

        $ctx = $storageAccount.Context
        $containers = Get-AzStorageContainer -Context $ctx

        foreach ($container in $containers) {
        $results += [PSCustomObject]@{
            SubscriptionId       = $subscription.Id
            SubscriptionName     = $subscription.Name
            ResourceGroup        = $storageAccount.ResourceGroupName
            StorageAccount       = $storageAccount.StorageAccountName
            ContainerName        = $container.Name
            UsedCapacityInBytes  = $averageCapacity
            TagName              = $tagKey
            TagValue             = $storageAccount.Tags[$tagKey]
        }
      }
    }
}

# Output the results
$results | Format-Table -AutoSize

Output:

SubscriptionId                       SubscriptionName                      ResourceGroup StorageAccount ContainerName UsedCapacityInBytes TagName   TagValue
--------------                       ----------------                      ------------- -------------- ------------- ------------------- -------   --------
15xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxf xxxxxx xxxxxx xxxxxxxxxx Subscription venkatesan-rg venkat6781     data                     81178955 createdby venkat  
15xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxf xxxxxx xxxxxx xxxxxxxxxx Subscription venkatesan-rg venkat6781     test                     81178955 createdby venkat  
15xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxf xxxxxx xxxxxx xxxxxxxxxx Subscription venkatesan-rg venkat891      results                   7577683 createdby venkat  
15xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxf xxxxxx xxxxxx xxxxxxxxxx Subscription venkatesan-rg venkat891      sample                    7577683 createdby venkat  

Reference:
PowerShell script to fetch Azure storage account based on tag name & value – Stack Overflow by me.