I am in college admissions and here's the down low.
Few colleges require the class rank and size at application time because many high schools don't rank their students and because there are many homeschooled students who (for obvious reasons) aren't ranked. (A student once put "1/1 - duh" under "rank/class size" on his application, to emphasize that point.)
If the student has a class rank to be proud of (read: top 10%,) report it on the app and transcripts. Otherwise, just be prepared to answer the question during an interview because if the school does rank, the student should know his/her place in the ranking.
"My school does not rank" is a perfectly valid response, as is "I am a homeschool student." At that point, it's mainly to gauge how the student addresses the missing information and determine if it's a concern. As someone else pointed out, the admissions counselor for the territory likely knows the caliber of students at a given HS. Being in the top 50% at a very-competitive, high-achiever school carries more weight than being in the top 50% at a less-challenging school.
Rank is never the only deciding factor for college admissions, so don't sweat it too much.
Public Service Announcement: Please make sure your students use the same email, home phone and given/family names if you want to have a smooth admissions process. It's unbelievable how many applicants use random email addresses - one student used a different one for every step of the process. (Inquiry card, application, test scores, interview, etc.) The nicknames like "Jimsy" don't match automatically to "James" so the staff has to hunt for the correct documents to match up. Don't even get me started on the pretentious suffices, like "James Earl Jones XIV" like they're royalty. Your family name is JONES, not JONES XIV. Gak.
Set up a special email account when they're sophomores and make sure they use the same names on everything, including college fair registrations.