Available Disk Space

Not sure exactly what you mean by "Windows Installer Editor".

If you have a functional computer just open My Computer and right click on the drive in question and go to properties.

If you are trying to install windows on a PC then there should be a screen after you click Install that will let you select the drive/partition you want windows installed on. That screen also shows the capacity and available space of each drive.

I cannot tell what is "windows installer editor", but if you meant "Windows installing environment", this is very simple.

At the end of loading it's whole set of drivers, the system prompts you to press F8, and to accept EULA - you are asked to choose the installment options (available drive and partition), following which you may define the size of your desired partitions - according to the disk-size determined by the system.

In other words: the system analyses the available space for the installation of windows, then it reports the available size to be lated dedicated for the installation of windows - by means of formatting the disk.

After Windows is installed, you have "My Computer" icon on your desktop or within the "Start" menu.

There you may see the recognisable drives or partitions, and when you right-click a drive, partition or directory, and choose "Properties" - you have an updated estimation of available and occupied space - Windows Directory included.