You can use Get-AzureRoleSize cmdlet to get virtual machine sizes. Since there are many sizes available, you can use Format-Table cmdlet to display in a nice tabular format:

There are much more role sizes available then shown in the image. The Get-AzureRoleSize cmdlet returns all of the available sizes for virtual machines and
cloud services (web and worker roles) if the InstanceSize parameter is not specified. If you
specify a size, the cmdlet only returns information for that size. This cmdlet also exposes two properties: SupportedByWebWorkerRoles and SupportedByVirtualMachines. These properties identify whether the size is supported on web and worker roles, virtual machines, or both.

You can pipe output to Select-Object and Sort-Object cmdlets to filter and sort per your requirements. For example, to get role sizes, sorted by available RAM on servers, use below command:

Get-AzureRoleSize | select InstanceSize,Cores,MemoryInMb | Sort-Object MemoryInMb -Descending

To get role sizes, sorted by maximum number of data disk available, use below command:

Get-AzureRoleSize | select InstanceSize,MemoryInMb,MaxDataDiskCount | Sort-Object maxdatadiskcount -Descending

You can also identify the currently available virtual machine configurations through MSDN at http://msdn.microsoft.com/en-us/library/azure/dn197896.aspx