I have a box of 8mm round x 3mm thick neodymium magnets and a box of 8mm round x 1mm thick neodymium magnets. Originally, I got the 3mm thick ones for a print I got of thingiverse and then the 1mm thick ones for some stuff I waned to do. Overall, I use the 1mm thick ones more. But the 3mm ones are nice when I want a solid connection. That they are 8mm in diameter is probably less important than just having standard sizes I can design around. So, I’d say have some standard magnets you can design around. The exact sizes are probably less important; but, knowing you have them, you can plan for them.