Assign Variable From SQL

I have a table set up like this:

tbl_original
account, invoice
11111, 1
22222, 1
33333, 5
44444, 3

I want to create a function module or something similar which will assign a variable to the max invoice # used on that table, using sql if possible. I tried something like this, but it's not working.

Public Function max()

Dim maxinvoice As String

maxinvoice = "SELECT Max(tbl_original.invoice) AS MaxOfinvoice" & _
"FROM tbl_original;"

MsgBox maxinvoice

End Function

I would like the msgbox to popup saying "5", since that is the max invoice # used based on the above table.

I don't have too much experience using modules on their own without forms and variables on the form to assign to, can someone make a suggestion?

Thanks in advance!

Vassago