What are you trying to achieve particularly? You cannot easily achieve custom inline text size which means the question is why you would want to do that. There are various options but they make your notes non portable.
You can use existing or custom inline syntax to mark your content and then style it using CSS. For example you can use CSS to style highlighting == ⋯ == however you want (e.g. make it look smaller).
Or you can use CSS classes.
This is a normal text and this is <span class="small-text">smaller text</span> in the same line.
.small-text {
font-size: 0.75em; /* Adjust the size as needed */
color: #555; /* Optional: Change color */
}
Third option is using small tag or some other existing html tag. In any case you need to use a special syntax in your notes. There are various ways to automate this process like Raycast/snippets.