Open & Launch the OST Recovery Tool. Click Start » Search OST Recovery Tool
To select the OST file, click on ADD File button as highlighted below
As you click on Add file button, a small window will pop up and you need to browse the OST file location from machine.
Select any one of the OST files from the folder. The size of OST file has no limitation as you can see in image that the selected OST file is quite large in size.