1) you can concatenate in tsql with + sing
2) why don't you concatenate in C#?
3) use parameters for statement building (avoid exposing your app to SQLinjection)
cmdupdate.CommandText = "Insert Into RawMat (ProductDimension) VALUES (@dim)";
cmdupdate.Parameters.AddWithValue("@dim", string.Format("{0} x {1} {2}", txtwidth.Text, txtheight.Text, txtunitofmeasurement.Text));