If you are wondering how much the size of your script matters to PHP, you need not - PHP's script parser is incredibly fast, able to read in 100,000 lines of code in about three seconds on an 800MHz computer. Naturally it is generally not smart to have 100,000 lines of code in a script - you should try to split them up, for ease of maintenance if nothing else.
PHP code execution also is not affected by large scripts, although naturally its execution time will vary depending on how complex your code is. Reading and changing variables is almost instantaneous, whereas reading from and writing to files is one of the slower operations.
Remember that if you use a PHP code cache, PHP's parser has no work to do at all, so script size becomes even more irrelevant.