Document Lines Length - Enable As Float Integer?

I was working on editing a code for a simple text program, but when running the program for multiple lines, the text being read gets too long or too many lines, the computer slows down alot and almost stops functioning. I am using standard integers like document.lines.length or listbox.lines.count. I think these are standard integers and I need to know how to enable them as Long of Float Integers.

I'm assuming I just change
Dim X as integer to
Dim X as long
or Dim X as int16

But I'm not sure as everytime I change something I get cannot convert to double. The code I'm interested in changing is ...
For i As Int16 = 0 To ListBox1.Items.Count - 2

The list box will only get to about 3000 items or so.
and
For iPos = 1 To xy
If Mid(sString, iPos, Len(Text1)) = Text1 Or Mid(sString, iPos, Len(Text2)) = Text2 Then
JJ = TextBox1.GetLineFromCharIndex(iPos)
This read box will read documents above 32,000 lines, which is the reason for not wanting to use Integer.