IMHO, you shouldn't have to use scrollbar to read your function. As soon as you need to move scrollbar, it take a few more time to understand how work the function.
Accordingly, it depends of usual programming environment of your team work (screen resolution, editor, font size, etc...). In 80's, it was 25 lines and 80 columns. Now, on my editor, I display nearly 50 lines. Number of columns I display did not change since I split my screen in two to display two files at times.