Symbols scale with text size, you you could e.g. use
#set list(marker: (text(0.8em, sym.square.filled.tiny), [‣], [--]))
As a note, sometimes a symbol may be too small for your taste, but increasing its size affects line height against your wishes. In such cases, I have in the past put symbols in a box with fixed height:
#box(height: 0.65em, text(1.1em, sym.ballot))
The 0.65em here is not based on any specific principle but on trial and error; it matches the default par.leading, although I did not care to verify if that means anything. If the baseline is off after doing this, you can use move to further adjust.