I'm writing a program that fits into a client's work process. I cannot change what the client is doing...

The client is going to copy & paste my text file into excel. Is there a way to set the font size in a text file so when it gets pasted into excel that text is larger?

As far as I'm aware, Text files, by definition, contain no special formatting. You would have to use another format. For example, rich text file (rtf) that wordpad supports or Doc that ms word supports. If he wants to open it in a browser, you can use html.

Instead of Ctrl+V to paste you can try Ctrl+Alt+V to paste, it'll give you options "how" to paste the text (at least in MS word).

I've never had any need, or know whether it'll allow you to post text bigger,. unless you already set the text size in the XLS file to that size.
If so, you can paste while retaining the text size of the XLS document rather than pasting the style of the source document.

Why though?
Depending on what tool you're coding in, you could probably either generate an excel doc yourself, or insert the intended text into a template file,. why would you ask your client to copy paste text themselves?