6x10 would work. I mainly use a 6x3 and get three 7/8 blanks out of it. That big slab you saw I used a 8x8 brownie pan. just make sure you measure how much resin it will take to fill it up first. I do it by filling my pan with water and then pouring it into a measuring cup and thats how much resin to use. Besides that, getting all the colors to gel at the same time is the HARDEST thing to do. some colors tend to set faster, you could be wearing mix-matched socks that day, it doesn't matter....it's something trial and error can only teach. try a couple small pours to get your feet wet if this is your first time casting. Hope any of this helps and feel free to ask any questions. thats what we're here for!