btw, you already know how much space you got. ok? so i suggest that you put your total space in your web.config and then get sum of sizes of files already on the disk and subtract them.
long totoalSpace = long.MaxValue; // as example
long size = 0;
string folderPath = @"d:/hostingpace/yourwebsite/files"; //sample path
foreach (var item in Directory.GetFiles(folderPath))
{
size += new FileInfo(item).Length;
}
long remainingSpace = totoalSpace - size;
note that since you dont have access to phisyical path you should Server.MapPath to get the folder path..