fileUpload1.FileBytes.Length
will give you size in bytes
Example
int sizeinbytes=fileUpload1.FileBytes.Length;
also
you can try
int sizeinbytes= FileUpload1.PostedFile.ContentLength;
Updated to display size in user friendly manner:
string[] sizes = { "B", "KB", "MB", "GB" };
double sizeinbytes=fileUpload1.FileBytes.Length;
int order = 0;
while (sizeinbytes>= 1024 && order + 1 < sizes.Length) {
order++;
sizeinbytes= sizeinbytes/1024;
}
string result = String.Format("{0:0.##} {1}", sizeinbytes, sizes[order]);
[Edit - Just brought the OPs two answers into one.]
If you want to get file size through javascript,
then check this solution
File size detection[
^]