If I want to launch a Konsole can I do so with a font-size of 30px rather than the default?
Probably not - the KDE developers are dead set against that, per se. You can create a profile and reference that (see this).
As noted in a followup comment, if you happen to know the name of the relevant property, you can override that on the command-line for a given profile, as hinted in the documentation using
Change the value of a profile property.
These options help find the name:
List all available profiles.
List all possible properties with name and type. See option -p. For more information, please visit [Konsole API Reference].