I'm creating a product database and I need to store product length in decimal format (e.g. product with ID=25 has length=2.5)

I have the following code:
prod_length = CDbl(Request.QueryString("prodLength"))
' where prodLength is a floating point number

' then I send to stored procedure
Set prm = conn.CreateParameter("",adDouble , adParamInput,8,prod_length)
conn.Parameters.Append prm

In my stored procedure I define the prodLength value thusly:

but this just doesn't work. What two floating point assignments in vbscript and SQL are compatible?? I've tried using Single,Double,Numeric with float and numeric in SQL and it either does not work or (naturally) it strips the decimal numbers off and just gives me an int. What is the solution?