Best Way to use PHP code with WordPress Pages?

I am wondering what the “best practice” solution to the following problem is. Any advise you can give would be gratefully received.

I have a page that describes a program I am writing. Every time I release a new version of the program it is copied up to my webserver automatically. The page that describes the program dynamically determines the size of the file and displays it like so: “Boggle_0_3.zip (2.3MB)”. The “2.3MB” bit is determined by a bit of PHP every time the page is requested.

Now I have switched to WordPress I’d like the page describing the program and offering it for download to be a WordPress “page” to get the look and feel the same as everything else, etc. Except there’s no (simple) way for it to do the dynamic lookup of how large the files are even though it is a simple bit of PHP.

So I suppose my question is: 1. Can I somehow put PHP code into a WordPress page? And if not, 2. How do I hook into my WordPress look and feel from a PHP of my own creation?

Thanks for looking, Tom