Slightly faster:
[pascal]var
MsgTxtLength: Integer;
MsgTxt: Array[0 .. 255] of Char;
MyString: AnsiString;
...

BlockRead(FileStream, MsgTxtLength, SizeOf(MsgTxtLength));
BlockRead(FileStream, MsgTxt[0], MsgTxtLength); // Char is size 1 so length works
// Or into a string
BlockRead(FileStream, MyString[1], MsgTxtLength);
...
[/pascal]