Problem inserting a text file into an ACCESS DB

When I insert this text file into my db I asigned the size of each field this way:
date = Trim(Mid(LineaLeida, 1, 10))

But it gives me a problem when one particular field changes its length from time to time (it doesn´t have a constant length). I´d like a way of measuring the field´s length and taking as many characters as its length instead of taking some of the next field´s characters in order to complete its defined length, which is what´s happening now.

I know what I have to do but I´m not sure how to do it?, maybe someone could help me out on this one.

Thanks!

================================================== =================================================
This is part of my code:

NumFichero = FreeFile
Open Path For Input As NumFichero

Do

Line Input #NumFichero, LineaLeida
fecha = Trim(Mid(LineaLeida, 1, 10))
hora = Trim(Mid(LineaLeida, 12, 8))
usuario = Trim(Mid(LineaLeida, 21, 12))
codigo = Trim(Mid(LineaLeida, 34, 12))
cliente = Trim(Mid(LineaLeida, 47, 7))
cuenta = Trim(Mid(LineaLeida, 55, 7)) <=====================This is the field that changes its length
movil = Trim(Mid(LineaLeida, 63, 10))
ci = Trim(Mid(LineaLeida, 74, 2))
mes = Trim(Mid(LineaLeida, 77, 4))
estado = Trim(Mid(LineaLeida, 82, 31))

If fecha <> "Fecha" Then

query = "INSERT INTO log (fecha, hora, usuario, codigo, cliente, cuenta, movil, ci, mes, estado) VALUES ('" & fecha & "', '" & hora & "', '" & usuario & "', '" & codigo & "', '" & cliente & "', '" & cuenta & "', '" & movil & "', '" & ci & "', '" & mes & "', '" & estado & "') "
'txtfields1.Text = query
db.Execute (query)

End If

Loop Until EOF(NumFichero)

Close NumFichero

================================================== =================================================

There is a Len function in VBScript

numChar = Len(someString)

I tried the len function and worked like a charm.
Thanks Doug!