For a physical environment, you need to collect all the data from the different operating systems, and then analyze and aggregate them all to right-size your ESXi hosts.

Different tools could be used but, basically, you can use standard tools provided by the different operating system, for example, Performance Monitor in a Windows system and top, vmstat, sar, and other command-line tools in a Linux system. However, using those tools is time-consuming even in a relatively small data center, because acquiring meaningful and useful inventory information requires manual aggregation of the results, and the approach is impractical in medium-to-large data centers.

VMware provides different tools or services to provide support during the initial assessment of a physical environment, with the purpose of right-sizing the new virtual environment:

• VMware Capacity Planner: This can help you plan for capacity optimization and design an optimal solution to achieve maximum performance. Capacity Planner is an IT capacity planning tool that collects comprehensive resource utilization data in heterogeneous IT environments, and compares it to industry-standard reference data to provide analysis and decision support modeling. This tool is limited to VMware Professional Services Organization (PSO) or authorized partners.
  For more information refer to the service page at https://www.vmware.com/products/capacity-planner.html.

• SysTrack Desktop Assessment: Through the use of the VMware desktop assessment service, leveraging SysTrack technology, your IT administrators gain a comprehensive understanding of the current end user environment as you evaluate Windows 10 migrations. Details on how each VMware solution would be beneficial are identified and quantitatively analyzed, resulting in the insights necessary to feel confident in moving forward with Windows 10.
  For more information refer to the service page at https://www.vmware.com/files/microsites/latitude/index.html.

However, there are also other tools from different vendors; third-party workload-sizing tools can also be a valid option, but the prices and capabilities of these tools vary. For example, consider the following tools:

• Dell Performance Analysis Collection Kit (DPACK): This works non-disruptively in Windows, Linux, and VMware environments; customers typically allocate approximately 24 hours for data collection. The DPACK collector runs remotely and is agentless, gathering core metrics such as disk I/O, throughput, free and used capacity, and memory utilization. Then DPACK produces an in-depth analysis of server workloads and capacity requirements to help optimize data center operations and expansion.
  For more more information refer to the product page at http://www.dell.com/en-us/work/learn/dpack.
• Platespin ReCon: This was formerly known as NetIQ PlateSpin Recon. This is a virtualization planning tool for complex server consolidation and disaster recovery initiatives. It is really powerful and one of the first tools (excluding VMware Capacity Planner). It was also one the first to support multi-platform environments across Windows, Linux, and Unix servers.
  For more information refer to the product page at https://www.microfocus.com/products/platespin/recon/.
• Quest Foglight: This was formerly known as Vizioncore Foglight. This supports multiple hypervisors to drive your organization's virtual strategy across VMware, Hyper-V, and OpenStack. Additionally, Foglight can monitor Microsoft® Active Directory (AD), Exchange, and Office 365, as well as the storage part.
  There's more information on the product page: https://www.quest.com/products/foglight-for-virtualization-enterprise-edition/.

The most interesting products are VMware Capacity Planner and Dell DPACK.