Mail message font size

I am sending the contents of clipboard as an email.
I don’t understand why the message font size is very small. I am using this in my script:

property FontSize : 18
	set newMessage to make new outgoing message with properties {subject:theSubject, content:theBody, message font size:FontSize & return & return}

I don’t get any errors but the font size remains small.
What am I doing wrong here?

Secondly, would it be possible for the message to retain the size of the clipboard contents instead of 18. (I am not intending to open the compose window and keystroking cmd with V)
Thanks

This works.

tell application "Mail"
	
	set newMessage to make new outgoing message with properties {subject:theSubject, content:theBody & return & return}
	tell newMessage
		set size to 18
		set visible to true
		set sender to theSender
		make new to recipient at end of to recipients with properties {name:theName, address:theAddress}
		end tell

I guess, I must be wrong in interpreting the dictionary earlier.

But this is what i see in the Mail dictionary:

but the script below fails to change the font size

tell application "Mail"
	set message font size to 44
	set newMessage to make new outgoing message with properties {subject:theSubject, content:theBody & return & return}
	tell newMessage
		--set size to 72
		set visible to true
		set sender to theSender
		make new to recipient at end of to recipients with properties {name:theName, address:theAddress}
		end tell

message font size affects only the message viewer.
It’s the equivalent to the size of the setting Message Font in Preferences > Fonts & Colors