Are you referring to the PHP file size? That does not matter in download time of it's result. The main thing would be the size of the result it dishes out to the browser.
The system I'm involved in is very heavy on forms and on most of the screens when loaded, the file size can be up to 300-500kb at once and the clients are mostly using satellite analogue modems to grab these pages for data entry - yes it's dead slow (getting 16kbps would be a very good day) but it works.
And to put your worries down the core php script that executes on mine is about 2mb in size.