The standard resolution for most uses is 300ppi. To get a 8x8' image, increase the resolution to say 4,800 but to process this monster to create a 6x9 or 10x15 print you would have to process your file with Photoshop or Lightroom or something similar. Say you have your slide scan set to lifesize 100%. Do you mean you want to scan a say 2x2' mounted slide and scan it to a 8x8' digital image file? I do not think that can be done directly. Rereading and thinking about your question more.

Somehow I need to get from Thumbnails to full scan.Using Epson Scan 2, This screen opens showing Thum. 'Next' takes me to this Save To Computer screen which just saves the Thumbnail. The Review Scans screen shows me the Previews or Thumbnails. When I click on Scan on the first screen, I get the third screen. I was looking for something that would let me go to the full scan. When I click on that, the second screen shows that I get this pop up allowing me to pick a cropped area size etc. Using Epson Scan 2, This screen opens showing Thumbnail options.

Epson perfection v500 vs v600