From Xojo Documentation

Property (As UInt64 )
aBinaryStream.Length = newUInt64Value
UInt64Value = aBinaryStream.Length

Supported for all project types and targets.

The length of the file in bytes. If you set the Length property to a value smaller than its current value, it will truncate the file.

Sample Code

Var i As UInt64
Var readStream As BinaryStream = BinaryStream.Open(readFile, False)
i = readStream.Length