The first question is interesting and I don't know. As to the second, it has to do with the button argument. See your helpfile 'msgbox'for the different types. For the critical, here the code.
value = msgbox("whatever",vbCritical)
or
value = msgbox("whatever",16)
Msgbox font size is controled via your systems
Display | Proerties | appearance
ie. Right click desktop
select properties
select appearance
select Item - messagebox
Ivan
Mark, just in case you don't know, this will change your message box properties for the entire system, not just in Excel.