@JayantaPoddarhas what is essentially the only workaround at the moment. Your situation of wanting to change the label's font size is easily accomplished there, but labels do not currently support property connections for any label properties besides what can be done in the "<FNT>" tag.
If you think it would be helpful to simply peg a label property to an attribute or expression, I'd appreciate a vote in favor of my suggestion to implement this.
- Josh Carlson
Kendall County GIS