I need a way to get a list of available drives and their size while in WinPE.

If you need to simply list the drives, you may use WMIC.  Windows Management Instrumentation (WMI) is the infrastructure for management data and operations on Windows-based operating systems.

Bring up the command prompt and type the following command:

    wmic logicaldisk get name

Press Enter and you will see the list of Drives.

You can also use the following parameter:

    wmic logicaldisk get caption

Using the following will display Device ID and volume name as well:

    wmic logicaldisk get deviceid, volumename, description

You can also use diskpart to get a list of drives along with some more details. The  Diskpart utility can do everything that the Disk Management console can do.

Bring up the command prompt and type:

    Diskpart

This will load Diskpart and you can then get a list of drives with their size and volume label:

    list volume

You can also use Gdisk to get a list of drives along with some more details.

Gdisk commands