It will take time to figure out your preferences. I personally benefited from buying various small quantities to see what I liked and what worked well. Now I just keep a bunch of sizes on hand at all times b/c it's nice to have variety for different applications. Clear is cheap and you'll use it ALL in the beginning, whatever your purchase. No need to buy cases right off the bat, experiment and see what you like/want. You also might end up switching gears and changing from pipes to marbles (as an example).

For me, this is always stocked:
4mm
6mm
8mm
10mm
16mm (for pendants)
12x2 - blowtubes
16x4
25x4 - great for learning how to work hollow

Good luck and have fun