you could easily add custom styles to any html tag
## this is a title and <span style="font-size: .5rem">this should be small</span>
this is a title and this should be small
but as you see it doesn't work here
inline styles are valid markdown but they don't work here or any stackexchange sites.
you could instead use
<sub> to make things a bit smaller but with no flexibility
## this a title and <sup>this should be smaller</sup> <sub>or like this</sub>
this a title and this should be smaller or like this
you can chain these tags to achieve your desired effect
## this a title and <sub><sup><sub>this should be smaller<sub/><sup/><sub/>
this a title and this should be really small!