You will need to do some work to better judge this issue. Remember that in order to take advantage of the SLA for Windows Azure Compute, you will need at least 2 instances (so now you have instances in different error domains, so your site stays on even if one of the instances is processed due to an OS update, a hardware failure etc.). The question then becomes: can two copies of Extra Small process 20,000 views daily? This is equivalent to approx. 10 Kbps per virtual machine instance per day, or 416 views per hour, or 7 per minute. And ... even with one instance, the hit rate of 14 per minute is pretty low.
More than a processor, you may run into bandwidth because you will only see about 5 Mbps per instance versus about 100 Mbps per small instance.
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EDIT ( 2012 .): 0,02 ./ 0,04 . , 6 XS Small. XS . . ( ).