For a text field you cannot do that. Yu can create multiple text fields with different font sizes that overlay each other and then conditionally suppress them.
For most other field types you right click the field and select format field.
click on the Font tab.
Click on the formula button next to size and put your conditional formula there as "if condition then fontsize# else fontsize#"
Example: if {tbl.color}="red" then 10 else 8