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