According to https://github.com/MicrosoftDocs/vsts-docs/blob/master/docs/pipelines/agents/hosted.md the virtual machine size of the Microsoft-Hosted Agent is:
There isn’t the way to force build/release to use DS3_v2 vm agent, it usually try to get DS2v2 sku VM, but sometimes gets a DS3V2 based on availability. We recommend you to setup private agent if hosted agent cannot meet your requirement.