To everyone who replied, thanks. I will build a 36" x 48". Using high density polyethylene instead of granite eliminates the problem of finding, paying for and lifting the actual surface plate and size is whatever you order. I have had a piece of it in the shop for several years and can find zero warpage. Of course, you would not want to get it too hot.

So far, looking at the various retailer sites, I have not found a picture of how their surface plate is set up. There must be a post to set the BB Shell on. I would love to see a pic. I assume the BB shell is never moved but the rest of the frame can be adjusted relative to it. Is this the case or do I have it all wrong? Jerry