Typically dado stacks come in 2 sizes, 6" and 8". If your going to plow 1/4" dados, rabbits and grooves, a 6" will suit you fine. That's what I did, but I didn't plan for the future. I built a box joint jig, and the 6" wasn't big enough. I actually just bought an 8" set.
With that said, your saw may not be powerful enough. The arbor may not have the space to accommodate a big dado stack. Definitely upgrade your saw when you can.
With a dado stack, you have a lot of steel spinning at once, can do a lot of damage to flesh. Use push blocks to push the material across the blades, never just your hands.