Can we setup auto scale in azure cloud service sizes? for example in day time i want the large(7gb ram) and night time i want it to be medium(3.5 gb ram) size.
There is a option for increasing and decreasing the cloud service instance size.
AFAIK, it is not possible to achieve through auto scaling because the VM size is defined in the csdef file that gets bundled into the cloud service package file. In order to achieve this, you would need to redeploy the package which is not possible through auto scaling.
However you could write some PowerShell scripts and run them from within your infrastructure on a scheduled basis that can accomplish the same.