If your current wheels are the standard 17x7 at 55mm offset, these new wheels would sit:

55-35 = 20mm further out through offset.

9.5-7 = 2.5 inches wider.

2.5×25/2 = 31.25mm further out through width.

Total 51.25mm further out than the current wheels. If your arches are a full 2 inches proud of the current wheels then the sizes are probably right.

18s will look good but require a lower profile tyres that might be a bit crashy over bumps, check the weight too as 18s tend to be a bit heavier and Miatas aren't keen on extra unsprung mass. Straight line speed also suffers due to extra rotational mass so acceleration will be reduced.

9.5 inch wide wheels may also need a massive tyre unless you are stretching a smaller width tyre on an oversize rim, in which case ride quality and handling play second fiddle to looks.