You asked about tray sizes. Normally the tray sizes account for the need to be able to sit the paper in the liquid, move it around and then lift it out so a 5x7 tray is a little bigger than 5x7, likewise 8x10. If you are doing both sizes you can simply use the 8x10, it saves buying a 5x7 set but at the cost of making up more liquid. The rate of usage rate should be the same so you can do twice as many 5x7s as 8x10s. Personally I'd go for the bigger trays which gives more room in each.