Yes you will be able to run Sitefinity in Extra Small instance. The requirements for Sitefinity in Azure are the same as the default System requirements
which the Extra Small instance covers.
As Matt mention the Azure instance will really require a lot of time to warmup, but this is true even in the small and medium instances. Sitefinity is not incredibly fast in the cloud as the heavy backend operations that will be improved really very soon with the realse of Sitefinity 5.1
which improve the backend performance gradually, but the warmup of the application pool will not benefit much from it.
All the best,
the Telerik team
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