Welcome to CNCzone! Take a look at Swede's (one of our members) web site. In his mill build he details in laymans terms the differences between the different grades of THK rails. Some accept loads in certain directions, others in all directions. In general the parts are NOT interchangable: you can't swap the slides off of one type of rail onto another. Beyond that rail size is pretty much directly related to load. You can, with a little care, butt two or more sections end to end. This is handy to remember as long rails on eBay are somewhat rarer and command top dollar. I've got 25mm HSR rails for a similar sized table (not yet built) that can handle wood and probably aluminum. If your definately thinking aluminum it might pay to step up to 35mm. Deals are funny: I've got some mongo 80" rails (think 1/4 scale train track) that cost less than the 25mm. Every deal is different! Good luck.