SQLClient SQLAdapter Data type problem

I'm having problems simply retrieving values from the database and using
in their original format. Example below assumes that size column is
double or float
Here is code snippet:

string maxSize;
string source = "Data Source=server_name; Initial Catalog=testing; User
ID=bj; Password=pass";
SqlConnection conn = new SqlConnection(source);
string select = "select max(size) from sizes";
SqlAdapter da new SqlAdapter(select, conn);
DataSet ds = new DataSet();
da.Fill(ds, "maxSize");

foreach(DataRow in ds.Tables["maxSize"].Rows)
{

maxSize = row[0].ToString();
}

Everything works here but how I go about it if I need maxSize to be
double or float? if I do maxSize = (double)row[0]; I get "cannot
explicitly convert object to double" There is something I don't quite
understand. And if query returns only one row, how can I get the size
value without using foreach?

Thanks in advance
.

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