How to make letters bigger in MsgBox() prompt?

Right-click on your desktop and select Properties. Choose the 'Appearance' tab and change the 'Item' dropdown to read Message Box. Change the font and style to your liking. I don't know if this can be done within Excel.

If you want to do this within excel then
try this;

Option Explicit
Dim newHour, newMinute, newSecond, waitTime

Sub ChangeMsg_FontSize()

Call Shell("rundll32.exe shell32.dll,Control_RunDLL desk.cpl,,2")
newHour = Hour(Now())
newMinute = Minute(Now())
newSecond = Second(Now()) + 3
waitTime = TimeSerial(newHour, newMinute, newSecond)
'Give time to get it up
Application.Wait waitTime

'Select msgbox from Properties | Appearance box
SendKeys "{TAB}{TAB}{TAB}{down}{down}{down}{down}{down}{down}{down}", True
'select font = same font-next size up
SendKeys "{TAB}{TAB}{down}", True
'select Apply button
SendKeys "{TAB}{TAB}{TAB}{TAB}{TAB}{TAB}{enter}", True
'now enter to dismiss
SendKeys "{enter}", True
'Now test via msgbox
MsgBox "Testing Size of Font"
End Sub

Sub ResetMsg()
Call Shell("rundll32.exe shell32.dll,Control_RunDLL desk.cpl,,2")
newHour = Hour(Now())
newMinute = Minute(Now())
newSecond = Second(Now()) + 3
waitTime = TimeSerial(newHour, newMinute, newSecond)
'Give time to get it up
Application.Wait waitTime

'Select msgbox from Properties | Appearance box
SendKeys "{TAB}{TAB}{TAB}{down}{down}{down}{down}{down}{down}{down}", True
'select font = same font-next size up
SendKeys "{TAB}{TAB}{up}", True
'select Apply button
SendKeys "{TAB}{TAB}{TAB}{TAB}{TAB}{TAB}{enter}", True
'now enter to dismiss
SendKeys "{enter}", True
'Now test via msgbox
MsgBox "Testing Size of Font"

End Sub

Ivan

...and i thought that it is so simple.....

Thank you and much appreciation
Eli