How to Set Max File Size for Upload?

I used to use <input type="hidden" name="MAX_FILE_SIZE" value="100000" /> in my PHP days, but it doesn't seem to work with Rails. I always thought MAX_FILE_SIZE was an HTML spec, but upon a little research, it looks like it is PHP specific. Can anyone confirm?

Anyway, I want to prohibit a user from uploading large files. For instance, the user has the option to upload 10 images, each to be less than 100K (1MB total).

-pachl