Check size of uploaded file in mb

Since you are given the size in bytes, you need to divide by 1048576 (i.e. 1024 * 1024):

var fileSize = imageFile.ContentLength;
if ((fileSize / 1048576.0) > 10)
{
    // image is too large
}

But the calculation is a bit easier to read if you pre-calculate the number of bytes in 10mb:

private const int TenMegaBytes = 10 * 1024 * 1024;


var fileSize = imageFile.ContentLength;
if ((fileSize > TenMegaBytes)
{
    // image is too large
}

You can use this method to convert the bytes you got to MB:

static double ConvertBytesToMegabytes(long bytes)
{
    return (bytes / 1024f) / 1024f;
}

Prefixes for multiples of bytes (B):
1024 bytes = 1kilobyte
1024 kilobyte = 1megabyte

double ConvertBytesToMegabytes(long bytes)
{
    return (bytes / 1024f) / 1024f;
} 

var fileSize = imageFile.ContentLength;

if (ConvertBytesToMegabytes(fileSize ) > 10f)
{
    // image is too large
}