In many cases it is a bad idea to use a ring terminal that is not exactly the same size as the stud.  A lot of sites tell people to never do this.

For example, on some of the Victron fuse blocks, the metal bar is only the same width as an M8 ring.  An M10 ring will extend over the sides of the bar and the surface area in common is reduced.

On the other hand, on some devices like the 3P 15k Voltronic inverters, they put an M8 screw into a very fat metal block.  The metal block is 30mm wide and 40mm long.  The block is bigger than an M8 ring, in fact it is bigger than an M10 or even an M16 ring.  If I put an M10 ring terminal onto that block then all the underside of the ring terminal will be in contact with the block.

Voltronic recommends 7-12 Nm torque.  To apply that torque equally across all of a ring terminal, it might be useful to include an extra-wide washer with M8 hole in it above the ring terminal.

How do people feel about mixing and matching sizes in cases like this where the ring terminal appears to be completely in contact with the block underneath?

I follow the same method as above. As my battery cable were used on lead acid with M10 studs I just reused them. They remain there if I have to use them on LA again should I have to fall back to 12V inverter and LA batteries in case of repair needed.